Metabolically Engineered Bacterial Strains Having Non-Functional Endogenous Gluconate Transporters

ABSTRACT

The present invention relates to engineering metabolic pathways in bacterial host cells which results in enhanced carbon flow for the production of ascorbic acid (ASA) intermediates. In particular, the invention relates to increasing the production of ASA intermediates in bacterial cells by enhancing the availability of gluconate resulting from the inactivation of endogenous gluconate transporter genes.

The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/473,310, filed May 22, 2003 and to U.S. Provisional Patent Application Ser. No. 60/478,056, filed Jun. 11, 2003.

FIELD OF THE INVENTION

The present invention relates to engineering metabolic pathways in bacterial host cells which results in enhanced carbon flow for the production of polyols and keto-derivatives thereof. More specifically the invention relates to the enhanced production of keto-polyols, such as sugar-keto acids and more specifically to ascorbic acid intermediates. In particular, the invention relates to enhancing the industrial production of gluconate, 2-keto-D-gluconic acid (KDG or 2-KDG), 2,5-diketogluconate (DKG or 2-DKG) and 2-keto-L-gulonic acid (KLG or 2-KLG) in Pantoea cells by altering an endogenous gluconate transporter. The invention further relates to altered Pantoea strains having one or more non-functional endogenous gluconate transporter genes and optionally inactivated endogenous glucokinase and/or gluconokinase genes.

BACKGROUND OF THE INVENTION

Numerous products of commercial interest, such as intermediates of L-ascorbic acid, have been produced biocatalytically in genetically engineered host cells. L-Ascorbic acid (vitamin C, ASA) is commonly used in the pharmaceutical and food industries as a vitamin and antioxidant, and due to this relatively large market volume and high value as a specialty chemical the synthesis of ASA has received considerable attention.

A chemical synthesis route from glucose to ASA, commonly known as the Reichstein-Grussner method, was first disclosed in 1934 (Reichstein T. et al., (1934) Helv. Chim. Acta., 17:311-328 and Reichstein T. et al. (1933) Helv. Chim. Acta. 16: 561, 1019). A bioconversion method for the production of an ASA intermediate, 2-KLG, has been disclosed by Lazarus et al. (1989, “Vitamin C: Bioconversion via a Recombinant DNA Approach”, GENETICS AND MOLECULAR BIOLOGY OF INDUSTRIAL MICROORGANISMS, American Society for Microbiology, Washington D.C. Edited by C. L. Hershberger). This bioconversion of a carbon source to KLG involves a variety of intermediates, and the enzymatic process is associated with co-factor dependent 2,5-DKG reductase activity (DKGR). Additionally, recombinant DNA techniques have been used to bioconvert glucose to KLG in Erwinia herbicola in a single fermentative step (Anderson, S. et al., (1985) Science 230:144-149). Effective procedures for converting KLG to ASA are described in Crawford et al., ADVANCES IN CARBOHYDRATE CHEMISTRY AND BIOCHEMISTRY, 37:79-155 (1980).

A number of strategies have been explored in bacteria and other microorganisms to increase the production of ASA intermediates. Some of these strategies include; gene deletions, gene additions and random mutagenesis. In particular, some gene manipulation strategies are mentioned below and include: (i) overexpressing particular genes in the ASA pathway (Anderson et al., (1985) Sci. 230:144-149; Grindley et al., (1988) Appl. Environ. Microbiol. 54: 1770; and U.S. Pat. No. 5,376,544); (ii) mutating genes encoding glycolytic enzymes (Harrod, et al. (1997) J. Ind. Microbiol. Biotechnol. 18:379-383; Wedlock, et al. (1989) J. Gen. Microbiol. 135: 2013-2018; and Walsh et al. (1983) J. Bacteriol. 154:1002-1004); (iii) utilizing bacterial host strains deficient in glucokinase (Japanese patent publication JP 4267860; Russell et al. (1989) Appl. Environ. Microbiol. 55: 294-297; Barredo et al. (1988) Antimicrob. Agents-Chemother 32: 1061-1067; and DiMarco et al. (1985) Appl. Environ. Microbiol. 49:151-157); (iv) reducing metabolism of glucose or gluconate by deletion of a gene required for phosphorylation, for example deleting the glucokinase gene (glkA) or gluconokinase gene (gntK) (WO 02/081440); and (v) reducing metabolism by manipulating enzymes involved in carbon utilization downstream of the initial glucose phosphorylation, for example manipulating phosphoglucose isomerase, phosphofructokinase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase or 6-phosphogluconate dehydratase.

Despite the above strategies, problems still persist concerning the diversion of ASA intermediates for catabolic metabolic purposes, and this results in reducing the efficiency and overall production of ASA intermediates. Thus, there still remains a need for improved production methods for ASA intermediates which are coupled to the metabolic pathways of host cells.

Carbon sources, such as glucose and gluconate, involved in the production of ASA intermediates may be separated by a cellular membrane from the reactions which utilize these substrates. When such a substrate and the synthetic machinery are separated, production of the product may require translocation of the substrate to the site of the synthetic reaction. Alternatively products generated inside the cell may require translocation from within the cell. Therefore, altering a substrate transport system may result in increased or decreased substrate availability for a particular metabolic pathway.

The present invention provides altered bacterial strains which include non-functional gluconate transporter molecules. The altered bacterial strains, which include the non-functional transporter molecules, have an increased amount of carbon substrate, such as glucose that may be utilized for the production of desired products, such as ASA intermediates.

SUMMARY OF THE INVENTION

The present invention provides methods of producing a desired product which comprises manipulating bacterial host cells to reduce carbon substrate diverted to metabolic pathways, thus increasing the productivity of the host cell for a desired product. Such manipulated bacterial host cells when cultured in the presence of a carbon source demonstrate increased yield of a desired product as measured directly and/or indirectly.

In one aspect, the invention provides isolated polynucleotides encoding polypeptides isolated from a strain of Pantoea having gluconate transporter activity. In one embodiment, the isolated polynucleotide comprises a nucleotide sequence having at least 40% sequence identity to a nucleotide sequence selected from the group consisting of SEQ ID NO: 1 and SEQ ID NO: 3. In a second embodiment, the isolated polynucleotide comprises a nucleotide sequence having at least 80% sequence identity to a nucleotide sequence selected from the group consisting of SEQ ID NO: 1 and SEQ ID NO: 3. In a third embodiment, the isolated polynucleotide comprises the nucleotide sequence of SEQ ID NO: 1 or SEQ ID NO: 3. In a fourth embodiment, the isolated polynucleotide comprises a nucleotide sequence which encodes a gluconate transporter having an amino acid sequence at least 40% identical to the sequence of SEQ ID NO: 2. In a fifth embodiment, the isolated polynucleotide comprises a nucleotide sequence which encodes a gluconate transporter having an amino acid sequence at least 80% identical to the sequence of SEQ ID NO: 2. In a sixth embodiment, the isolated polynucleotide comprises a nucleotide sequence which encodes a gluconate transporter having an amino acid sequence at least 80% identical to the sequence of SEQ ID NO: 4. In a seventh embodiment, the isolated polynucleotide comprises a nucleotide sequence which encodes a gluconate transporter having an amino acid sequence at least 90% identical to the sequence of SEQ ID NO: 4. In an eighth embodiment the isolated polynucleotide encodes a gluconate transporter having the sequence of SEQ ID NO: 2 or SEQ ID NO:4.

In another aspect, the invention relates to an isolated gluconate transporter protein comprising an amino acid sequence shown in SEQ ID NO: 2 or an amino acid sequence having at least 80% amino acid sequence identity thereto. In one embodiment, the isolated gluconate transporter protein has been isolated from Pantoea. In a second embodiment, the isolated gluconate transporter protein has the sequence shown in SEQ ID NO: 2.

In a further aspect, the invention relates to an isolated gluconate transporter protein comprising an amino acid sequence shown in SEQ ID NO: 4 or an amino acid sequence having at least 80% amino acid sequence identity thereto. In one embodiment, the isolated gluconate transporter protein has been isolated from Pantoea. In a second embodiment, the isolated gluconate transporter protein has the sequence shown in SEQ ID NO: 4.

In another aspect, the present invention provides a method for enhancing the level of production of an ascorbic acid (ASA) intermediate comprising, obtaining an altered bacterial host cell which is capable of producing an ASA intermediate, wherein an endogenous gluconate transporter gene has been rendered non-functional, culturing the altered bacterial host cell under suitable culture conditions in the presence of a carbon source to allow the production of an ASA intermediate and obtaining the ASA intermediate, wherein the level of production of the ASA intermediate is enhanced compared to the level of production of the ASA intermediate in a corresponding unaltered bacterial cell grown under essentially the same conditions. In one embodiment of the process, the endogenous gluconate transporter has at least 40% sequence identity to the amino acid sequence of SEQ ID NO: 2 or at least 80% sequence identity to the amino acid sequence of SEQ ID NO: 4. In a second embodiment of the process, the bacterial host cell is altered to include two non-functional gluconate transporter genes. In a third embodiment of the process, the gluconate transporter gene is deleted from the host chromosome. In a fourth embodiment, the process further comprises the step of isolating the ASA intermediate. In a fifth embodiment, the process further comprises converting the ASA into a second product. In a sixth embodiment, the host cell is an Enterobacteriaceae cell. In a seventh embodiment of the process, the endogenous gluconate transporter gene encodes a protein having at least 80% sequence identity to the sequence of SEQ ID NO: 2 or SEQ ID NO: 4. In an eighth embodiment of the process, the gluconate transporter gene encodes a protein designated GntU having the amino acid sequence of SEQ ID NO: 2. In an ninth embodiment of the process, the gluconate transporter gene encodes a protein designated IdnT having an amino acid sequence of SEQ ID NO: 4. In a tenth embodiment of the process, the bacterial host cell includes an inactivated glucokinase (glk) gene and/or an inactivated gluconokinase (gntK) gene. In an eleventh embodiment, the invention concerns an altered bacterial host cell produced by said process.

In a further aspect, the invention provides a recombinant Enterobacteriaceae strain comprising one or more nonfunctional endogenous gluconate transporter proteins. In one embodiment, the one or more non-functional endogenous gluconate transporter proteins has an amino acid sequence of at least 80% sequence identity to the sequence of SEQ ID NO: 2 or SEQ ID NO: 4. In further embodiments of this aspect the recombinant Enterobacteriaceae strain further comprises an inactivated endogenous glucokinase gene, an inactivated endogenous gluconokinase gene, and/or an overexpressed or heterologous DKG transporter genes. In a preferred embodiment, the recombinant strain is a Pantoea or E. coli strain.

In yet another aspect, the invention provides a recombinant Pantoea cell comprising an endogenous gluconate transporter gene which has been inactivated, wherein said gluconate transporter gene prior to being inactivated encoded a gluconate transporter protein comprising the amino acid sequence shown in SEQ ID NO: 2 or an amino acid sequence having at least 40% sequence identity thereto, wherein said protein functioned by transporting gluconate across a Pantoea cell membrane into the cytoplasm. In one embodiment, the recombinant Pantoea cell will include an inactivated endogenous glucokinase gene and in another embodiment the recombinant Pantoea cell will include a heterologous DKG transporter gene.

In another aspect, the invention provides an altered Pantoea cell comprising an inactivated endogenous gluconate transporter gene, wherein said gluconate transporter gene prior to being inactivated encoded a gluconate transporter protein comprising the amino acid sequence shown in SEQ ID NO: 4 or an amino acid sequence having at least 80% sequence identity thereto, wherein said protein functioned by transporting gluconate across a Pantoea cell membrane into the cytoplasm. In one embodiment, the altered Pantoea cell will include an inactivated glucokinase gene and in another embodiment the altered Pantoea cell will include a modified DKG transporter gene.

In a further aspect, the invention provides a method of enhancing the production of an ascorbic acid (ASA) intermediate from a carbon source comprising, obtaining a Pantoea host cell comprising an endogenous gluconate transporter gene which encodes a gluconate transporter protein having an amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 4, or an amino acid sequence having at least 40% sequence identity to SEQ ID NO: 2 or SEQ ID NO: 4, altering the Pantoea host cell by rendering the gluconate transporter gene non-functional, wherein the altered Pantoea host cell is capable of producing an ASA intermediate in the presence of glucose, culturing said altered Pantoea host cell under suitable conditions in the presence of a carbon source, and allowing the production of an ASA intermediate from the carbon source, wherein the production of said ASA intermediate is enhanced in the altered Pantoea cell compared to the production of the ASA intermediate in an unaltered Pantoea host cell cultured under essentially the same culture conditions. In one embodiment of the method, the ASA intermediate is selected from the group consisting of gluconate, 2-keto-D-gluconate, 2,5-diketo-D-gluconate, 2-keto-L-gulonic acid and L-idonic acid. In a second embodiment, the ASA intermediate is further converted to erythorbic acid or tartrate. In a third embodiment, the gluconate transporter gene is gntU having the nucleic acid sequence illustrated by SEQ ID NO: 1 and/or idnT having the nucleic acid sequence illustrated in SEQ ID NO: 3.

In still another aspect, the invention provides a method of increasing the production of KLG in a Pantoea host cell which comprises obtaining a Pantoea host cell including a gluconate transporter gene which encodes a gluconate transporter having an amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 4 or an amino acid sequence having at least 40% sequence identity to SEQ ID NO: 2 or SEQ ID NO: 4; altering the Pantoea host cell by rendering the gluconate transporter gene non-functional, wherein the altered Pantoea host cell is capable of producing KLG in the presence of glucose; culturing said altered Pantoea host cells under suitable conditions in the presence of a carbon source; and allowing the production of KLG from the carbon source, wherein the production of said KLG is enhanced in the altered Pantoea host cell compared to the production of KLG in an unaltered Pantoea host cell cultured under essentially the same culture conditions. In one embodiment, the invention relates to the altered Pantoea cell obtained according to the above method. In further embodiments, the method comprises a Pantoea cell genetically engineered to include a non-functional glucokinase gene, a non-functional gluconokinase gene and/or overexpressed DKG transporter genes.

In still another aspect, the invention provides a method of increasing the production or availability of gluconate in an oxidative pathway of a Pantoea host cell which comprises obtaining a Pantoea host cell including a gluconate transporter gene which encodes a gluconate transporter having an amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 4 or an amino acid sequence having at least 40% sequence identity to SEQ ID NO: 2 or SEQ ID NO: 4; altering the Pantoea host cell by rendering the gluconate transporter gene non-functional, wherein the altered Pantoea host cell is capable of producing gluconate in the presence of glucose; culturing said altered Pantoea host cells under suitable conditions in the presence of a carbon source; and allowing the production of gluconate from the carbon source, wherein the production or availability of said gluconate in an oxidative pathway is enhanced in the altered Pantoea host cell compared to the production or availability of gluconate in an unaltered Pantoea host cell cultured under essentially the same culture conditions. In one embodiment, the invention relates to the altered Pantoea cell obtained according to the above method. In further embodiments, the method comprises a Pantoea cell genetically engineered to include a non-functional glucokinase gene, a non-functional gluconokinase gene and/or overexpressed DKG transport genes.

Another aspect of the invention provides a method of increasing the production of KDG in a Pantoea host cell which comprises obtaining a Pantoea host cell including a gluconate transporter gene which encodes a gluconate transporter having an amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 4 or an amino acid sequence having at least 40% sequence identity to SEQ ID NO: 2 or SEQ ID NO: 4; altering the Pantoea host cell by rendering the gluconate transporter gene non-functional, wherein the altered Pantoea host cell is capable of producing KDG in the presence of glucose; culturing said altered Pantoea host cells under suitable conditions in the presence of a carbon source; and allowing the production of KDG from the carbon source, wherein the production of said KDLG is enhanced in the altered Pantoea host cell compared to the production of KDG in an unaltered Pantoea host cell cultured under essentially the same culture conditions. In one embodiment the invention relates to the altered Pantoea cell obtained according to the above method. In further embodiments, the method comprises a Pantoea cell genetically engineered to include a non-functional glucokinase gene, a non-functional gluconokinase gene and/or overexpressed DKG transport genes. In some embodiments, the invention includes the altered Pantoea host cells.

In still another aspect, the invention provides a method of increasing the production of DKG in a Pantoea host cell which comprises obtaining a Pantoea host cell including a gluconate transporter gene which encodes a gluconate transporter having an amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 4 or an amino acid sequence having at least 40% sequence identity to SEQ ID NO: 2 or SEQ ID NO: 4; altering the Pantoea host cell by rendering the gluconate transporter gene non-functional, wherein the altered Pantoea host cell is capable of producing DKG in the presence of glucose; culturing said altered Pantoea host cells under suitable conditions in the presence of a carbon source; and allowing the production of DKG from the carbon source, wherein the production of said DKG is enhanced in the altered Pantoea host cell compared to the production of DKG in an unaltered Pantoea host cell cultured under essentially the same culture conditions. In one embodiment the invention relates to the altered Pantoea cell obtained according to the above method. In further embodiments, the method comprises a Pantoea cell genetically engineered to include a non-functional glucokinase gene and/or a non-functional gluconokinase gene.

In yet another aspect, the invention provides a method for increasing the availability of gluconate for oxidative pathway production of an ascorbic acid (ASA) intermediate which comprises obtaining an altered bacterial host cell, wherein an endogenous gluconate transporter gene has been rendered non-functional; culturing the altered bacterial host cell under suitable culture conditions in the presence of a carbon source to allow the production of an ASA intermediate; and obtaining the ASA intermediate, wherein the ASA intermediate is selected from the group consisting of 2-keto-D-gluconate (2-KDG), 2,5-diketo-D-gluconate (2,5-DKG), 2-keto-L-gulonic acid (2-KLG), 5-keto-D-gluconate (5-KDG) and L-idonic acid (IA) and the endogenous gluconate transporter gene encodes a gluconate transporter having the amino acid sequence of SEQ ID NO: 2, SEQ ID NO: 4 or a sequence having at least 80% identity to either SEQ ID NO: 2 or SEQ ID NO: 4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the oxidative pathway for the production of ascorbic acid (ASA). E1 stands for glucose dehydrogenase (GDH); E2 stands for gluconic acid dehydrogenase (GADH); E3 stands for 2-keto-D-gluconic acid dehydrogenase (KDGH); and E4 stands for 2,5-diketo-D-gluconic acid reductase (2,5 DKGR).

FIG. 2 provides a schematic representation of some of the metabolic routes involved in glucose assimilation in a bacterial host cell such as Pantoea citrea. The following abbreviations have been used in the figure and are applied throughout the disclosure: glucose dehydrogenase=(GDH); gluconic acid dehydrogenase=(GADH); 2-keto-D-gluconate=(2-KDG or KDG); 2-keto-D-gluconic acid dehydrogenase=(2-KDGH or KDGH); 2,5-diketogluconate=(2,5-DKG or DKG); 2,5-diketo-D-gluconic acid reductase=(2,5 DKGR); 2-keto-L-gulonic acid=(2-KLG or KLG); 2-ketoreductase=(2KR or KR); 5-ketoreductase=(5KR or KR); 5-keto-D-gluconate=(5-KDG); idonate dehydrogenase=(IADH); glucokinase=(GlkA or Glk) and gluconokinase=(GntK). Boxes labeled with a “T” represent putative transporters and boxes labeled with an “E” represent putative efflux systems. Boxes labeled with a “1” or “2” represent putative gluconate transporters according to the invention.

As observed in FIG. 2, in a microbial cell there are multiple connections between oxidative and catabolic pathways, such as the glycolytic pathway, the pentose pathway, the tricarboxylic acid (TCA) cycle pathway and ASA intermediate production pathway. As can be seen, the inactivation of a gluconate transporter (1) and (2) blocks entry of gluconate into the cytoplasm resulting in a decrease in the amount of gluconate available for phosphorylation by GntK in the pentose assimilation pathway and resulting in a decrease in the amount of gluconate available for enzymatic reduction by 5-KR or 2-KR in the cytoplasm. Further it can be seen that inactivation of a gluconate transporter (1) also blocks entry of idonate into the cytoplasm and hence results in more substrate being made available for production of ASA intermediates such as KLG.

FIG. 3 depicts a nucleic acid sequence (SEQ ID NO: 1) for a Pantoea citrea gluconate transporter gene (gntU).

FIG. 4 depicts an amino acid sequence (SEQ ID NO: 2) for a Pantoea citrea gluconate transporter protein (GntU).

FIG. 5 depicts a nucleic acid sequence (SEQ ID NO: 3) for a Pantoea citrea gluconate transport gene (idnT).

FIG. 6 depicts an amino acid sequence (SEQ ID NO:4) for a Pantoea citrea gluconate transporter protein (IdnT).

FIG. 7 is a general schematic illustrating the process and vectors used to inactivate a chromosomal gluconate transporter gene in a strain of Pantoea citrea by homologous recombination using a non-replicating R6K vector, wherein gntU is the gene encoding a gluconate transporter protein, gntK is the gene coding for glucokinase, NsiI is a restriction site, loxP is a recombinase recognition sequence and Cm^(R) is the gene for the selective marker chloramphenicol. Reference is also made to example 1.

FIG. 8 is a general schematic illustrating the process and vectors used to inactivate a gluconate transporter chromosomal gene in a strain of Pantoea citrea by homologous recombination using a non-replicating R6K vector, wherein idnD is the gene coding for idonate dehydrogenase, idnT is the gene encoding a gluconate transporter protein, NsiI and BglI are restriction sites, loxP is a recombinase recognition sequence and Cm^(R) is the gene for the selective marker chloramphenicol. The plasmid, pEkldnDTcat2 was used for the transformation of the Pantoea host cell. Reference is also made to example 2.

FIG. 9 depicts the results from gluconate uptake rate experiments (nmole/OD/sec) for three different P. citrea strains. Strain 139-2A/pD92 was used as a control and was derived from ATCC® Accession No. 39140. Strain MDP41/pD92-A was derived from 139-2A/pD92 and includes an interruption of glkA. Strain MDP41UDT/pD92-A was derived from MDP41/pD92-A (according to the teachings of examples 1 and 2) and includes, in addition to an interruption of glkA, a gntU deletion and an idnT deletion.

FIG. 10 depicts the percent sequence identity between 10 gluconate transporters. The amino acid sequence as set forth in SEQ ID NO: 2 is designated Pc-gntU. The percent identity between Pc-gntU and the other gluconate transporters is 34% or less. The amino acid sequence as set forth in SEQ ID NO: 4 is designated Pc-idnT. The percent identity between Pc-idnT and the other gluconate transporters is 76.3% or less. The percent amino acid sequence identity between SEQ ID NO: 2 and SEQ ID NO: 4 is 33%. Gluconate transports Ec-yjhF, Ec-dsdX, Ec-gntP, Ec-gntT, Ec-gntU, Ec-idnT and Ec-0454 are obtained from E. coli and Bs-gntP is obtained from Bacillus.

FIG. 11 generally depicts some of the metabolic pathways involved in the synthesis of various polyols that are of commercial relevance.

DETAILED DESCRIPTION OF THE INVENTION

Recombinant bacterial host cells have been constructed which produce polyols, particularly keto-polyols and more particularly ASA intermediates, and which are altered to reduce carbon substrate diverted to catabolic pathways. Possible catabolic routes which can be used to divert glucose into cellular metabolism involving at least one enzymatic step, include but are not limited to the formation of each of the following: glucose-6-P, fructose, sorbitol, gluconate-6-P, idonate, sorbosone, deoxy-gluconate, 2,5-di-keto-gluconate, and 2-keto-D-gluconate, 5-keto-D-gluconate and 2-keto-3-deoxygluconate. (See FIGS. 1 and 2).

The practice of the present invention will employ, unless otherwise indicated, conventional techniques of molecular biology (including recombinant techniques), microbiology, cell biology, and biochemistry, which are within the skill of the art. Such techniques are explained fully in the literature, such as, MOLECULAR CLONING: A LABORATORY MANUAL, second edition (Sambrook et al., 1989) Cold Spring Harbor Laboratory Press; CURRENT PROTOCOLS IN MOLECULAR BIOLOGY (F. M. Ausubel et al., eds., 1987 and annual updates); OLIGONUCLEOTIDE SYNTHESIS (M. J. Gait, ed., 1984); PCR: THE POLYMERASE CHAIN REACTION, (Mullis et al., eds., 1994); and MANUAL OF INDUSTRIAL MICROBIOLOGY AND BIOTECHNOLOGY, Second Edition (A. L. Demain, et al., eds. 1999).

DEFINITIONS

Unless defined otherwise herein, all technical terms and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Singleton, et al., DICTIONARY OF MICROBIOLOGY AND MOLECULAR BIOLOGY, 2D ED., John Wiley and Sons, New York (1994) and Hale and Marham, THE HARPER DICTIONARY OF BIOLOGY, Harper Perennial, New York (1991) provide one of skill with general dictionaries of many of the terms used in this invention.

As used herein the term “polyol” means an alcohol molecule with numerous hydroxyl groups. A “keto-poly derivative” is a polyol which includes one or more keto groups in the polyol molecule. Examples of polyols include but are not limited to glucose, fructose, mannose, idose, galactose, sorbose, ribose, altose, arabinose, erythrose, xylose, xylulose, maltose, sucrose, erythrulose, gluconate, idonate, gulonate, galacturonate, mannitol, sorbitol, sorbosone, glycerol, erythritol, arabitol, DKG, KDG, KLG, gluconolactone and maltitol. The compounds may occur in either the D or L configuration.

As used herein an “ascorbic acid (ASA) intermediate” means a biochemical capable of being converted to ASA by enzymatic or chemical means and includes, but is not limited to, gluconate (GA); 2-keto-D-gluconate (2-KDG or KDG); 2,5-diketo-D-gluconate (2,5-DKG or DKG); 2-keto-L-gulonic acid (2KLG or KLG); 5-keto-D-gluconate (5-KDG) and L-idonic acid (IA).

As used herein, the term “carbon source” encompasses suitable carbon substrates ordinarily used by microorganisms, such as 6 carbon sugars, including but not limited to glucose (G), gulose, sorbose, fructose, idose, galactose and mannose all in either D or L form, or a combination of 6 carbon sugars, such as glucose and fructose, and/or 6 carbon sugar acids including but not limited to 2-keto-L-gulonic acid, idonic acid (IA), gluconic acid (GA), 6-phosphogluconate, 2-keto-D-gluconic acid (2 KDG), 5-keto-D-gluconic acid, 2-ketogluconatephosphate, 2,5-diketo-L-gulonic acid, 2,3-L-diketogulonic acid, dehydroascorbic acid, erythorbic acid (EA) and D-mannonic acid or the enzymatic derivatives thereof.

As used herein, the phrase “transport system” refers to at least one macromolecule, such as a protein, which is located in a cell membrane and is involved in the translocation of a chemical compound, such as a carbon substrate, across a cell membrane and into or out from a cell or cellular compartment. A transport system may include two, three, four, five, six or more macromolecules, such as proteins. These proteins may or may not be simultaneously induced by a substrate. (Reference is made to Saier M. et al., (1998) in ADVANCES IN MICROBIAL PHYSIOLOGY (Poole R. K. ed) pages 81-136, Academic Press, San Diego, Calif. for a discussion of the classification of transporters). In the literature, a transporter system is sometimes referred to as a permease. For example, the gluconate permease (GntP) family in E. coli is encoded by seven transporter genes which include gntP, gntT, gntU, gntW, ORFf449, dsdX and ORFo454 (Peekhaus et al., (1997) FEMS Micro. Lett. 147:233-238). Reference is also made to Yamada et al., (1996) Biosci. Biotech. Biochem., 60:1548-1550 and Tsunedomi et al., (2003) J. Bacteriol. 185:1783-1795. In one embodiment, a transporter protein as used herein is a “symporter”. A symporter is defined as a transporter that transports two or more substrates or species together in the same direction in a coupled process using an electrochemical potential gradient. One example of a symporter is a proton coupled substrate transporter.

The term “gluconate transporter” as used herein refers to a protein that catalyzes the transport of gluconate into the cytoplasm across the internal cell membrane. In addition to gluconate, a gluconate transporter may catalyze the transport of other sugar acid molecules or sugar-keto acids across a cell membrane.

A “sugar acid molecule” is defined as a sugar molecule having one or more acid groups. Preferred sugar acids that may be transported by a gluconate transporter according to the invention will not only include gluconate, but may also include idonic acid, galacturonic acid, glucuronic acid and/or gulonic acid.

A “sugar-keto acid” is defined as a sugar molecule having one or more keto groups and one or more acid groups. A gluconate transporter according to the invention may also catalyze the transport of sugar keto acids, such as 2-keto-D-gluconic acid (2-KDG or KDG), 2-keto-L-gulonic acid (KLG), 5-keto-D-gluconic acid (5-KDG), D-tagaturonic acid and/or D-fructuroinc acid.

The term “KDG transporter” as used herein, refers to a protein that catalyzes the transport of KDG across a cell membrane. More specifically the KDG transporter facilitates the entry of KDG into the cytoplasm across the internal cell membrane.

The term “glucose transporter” as used herein refers to a protein that catalyzes the transport of glucose across a cell membrane. More specifically the glucose transporter facilitates the entry of glucose into the cytoplasm across the internal cell membrane.

The term “DKG transporter” as used herein refers to a protein that catalyzes the transport of DKG across a cell membrane. More specifically the DKG transporter facilitates the entry of DKG into the cytoplasm across the internal cell membrane.

“Cytoplasm” or “cytoplasmic” refers to being within the inner cell membrane. Extracellular or outside the inner cell membrane refers to cell locations on the opposite side of a membrane from the cytoplasm, including but not limited to the periplasm. Internal or inner membrane (and sometimes referred to as the periplasmic membrane) refers to the barrier that separates the cytoplasm from the periplasm. Intracellular refers to the portion of the cell on the side of the membrane that is closest to the cytosol. Intracellular includes cystolic.

As used herein the term “gene” means a DNA segment that is involved in producing a polypeptide and includes regions preceding and following the coding regions as well as intervening sequences (introns) between individual coding segments (exons). As used herein when describing proteins and genes that encode them, the term for the gene is not capitalized and is italics, i.e. glkA. The term for the protein is generally not italicized and the first letter is capitalized, i.e. GIkA.

The terms “protein” and “polypeptide” are used interchangeability herein.

As used herein “oxidative pathway” of a host cell means that a host cell comprises at least one enzyme that oxidizes a carbon source, such as D-glucose and/or its metabolites. An oxidative pathway in a host cell may comprise, one, two, three or more enzymes. An example of an oxidative pathway is the formation of gluconate from glucose through the activity of glucose dehydrogenase. Another example of an oxidative pathway is the formation of DKG from glucose through the activity of glucose dehydrogenase, gluconate dehydrogenase and 2-KDG dehydrogenase.

As used herein “catabolic pathway” of a host cell means that a host cell comprises at least one enzyme that generates at least one metabolic intermediate. Very often, the formation of the metabolic intermediate is coupled to the generation of ATP, NADPH, or NADH for example by phosphorylating a carbon source such as D-glucose and/or its metabolites. An intracellular catabolic pathway in a host cell means the host cell comprises the activity of at least one enzyme in the host cell cytosol. In some embodiments, a catabolic pathway comprises the activity of two, three or more enzymes. Catabolic pathways include but are not limited to glycolysis, the pentose pathway and the TCA cycle pathway (FIGS. 1 and 2).

As used herein, the phrase “glucokinase” (E.C.-2.7.1.2) means an enzyme which phosphorylates D-glucose or L-glucose at its 6th carbon. As used herein, the phrase “gluconokinase” (E.C.-2.7.1.12) means an enzyme which phosphorylates D-gluconate or L-gluconate at its 6^(th) carbon.

The term “nonfunctional”, “inactivated” or “inactivation” when referring to a gene or a protein means that the known normal function or activity of the gene or protein has been eliminated or highly diminished. For example, inactivation of a gluconate transporter can be effected by inactivating the gntU and/or idnT chromosomal genes. Inactivation which renders the gene or protein non-functional includes such methods as deletions, mutations, substitutions, interruptions or insertions in the nucleic acid gene sequence.

A “deletion” of a gene as used herein may include deletion of the entire coding sequence, deletion of part of the coding sequence, deletion of the regulatory region, deletion of the translational signals or deletion of the coding sequence including flanking regions.

As used herein the term “mutation” when referring to a nucleic acid refers to any alteration in a nucleic acid such that the product of that nucleic acid is partially or totally inactivated. Examples of mutations include but are not limited to point mutations, frame shift mutations and deletions of part or all of a gene encoding a transporter.

As used herein, the term “modified” when referring to nucleic acid or a polynucleotide means that the nucleic acid has been altered in some way as compared to a wild type nucleic acid, such as by mutation in; deletion of part or all of the nucleic acid; or by being operably linked to a transcriptional control region.

An “altered bacterial host cell or strain” according to the invention is a bacterial cell having an inactivated gluconate transporter. In one embodiment, an altered bacterial host cell will have an enhanced level of expression (production) over the level of production of the same desired product in a corresponding unaltered bacterial host grown under essentially the same growth conditions. In other embodiments, an altered host cell will have an enhanced level of availability of gluconate to be used by the host cell to produce ASA intermediates as compared to a corresponding unaltered bacterial host cell.

An “unaltered bacterial host cell or strain” is a bacterial cell, which genetically corresponds to an altered bacterial host cell or strain but wherein the endogenous gene encoding the gluconate transporter encompassed by the invention is not inactivated and remains functional.

The “enhanced level of production” refers to an increased yield of the desired end-product, or amount of substrate as compared to the yield from an unaltered bacterial host when cultured under essentially the same conditions. For example an increase of 2%, 5%, 10%, 15%, 20%, 30%, 40% or more over the yield of the unaltered bacterial host. Yield, may be expressed in numerous ways including as a weight % (gm product/gm substrate) or gm/L per unit of time. The enhanced level of production results from the inactivation of one or more chromosomal genes encoding a gluconate transporter according to the invention. In a first embodiment, the enhanced level of production results from the deletion of one or more chromosomal genes encoding a gluconate transporter. In a second embodiment, the enhanced level of production results from the insertional inactivation (interruption) of one or more chromosomal genes encoding a gluconate transporter.

As used herein “chromosomal integration” is a process whereby an introduced polynucleotide is incorporated into a host cell chromosome. The process preferably takes place by homologous recombination.

As used herein, “modifying” the level of protein or enzyme activity produced by a host cell refers to controlling the levels of protein or enzymatic activity produced during culturing, such that the levels are increased or decreased as desired.

“Under transcriptional control” or “transcriptionally controlled” are terms well understood in the art that indicate that transcription of a polynucleotide sequence, usually a DNA sequence, depends on its being operably linked to an element which contributes to the initiation of, or promotes, transcription. “Operably linked” refers to a juxtaposition wherein the elements are in an arrangement allowing them to function. “Under translational control” is a term well understood in the art that indicates a regulatory process that occurs after the messenger RNA has been formed. “Expression” includes transcription and/or translation. The term “overexpressed” means an increased number of copies of the same gene in a host cell genome.

“Allowing the production of an ASA intermediate from a carbon source, wherein the production of said ASA intermediate is enhanced compared to the production of the ASA intermediate in a corresponding unaltered bacterial host cell or strain” means contacting the carbon source, with an altered bacterial strain to produce the desired end-product. By altering a gluconate transporter according to the invention, an altered bacterial host can demonstrate enhanced desired end-product production.

The phrase “desired end-product” or “desired product” as used herein refers to a desired compound to which the carbon substrate is bioconverted into. The desired end-product may be the actual compound sought or an intermediate along another pathway. Exemplary desired products are gluconic acid, 2-keto-D-gluconate; 2,5-diketo-D-gluconate; erythorbic acid; 5-keto-D-gluconate; 2-keto-L-gulonate; tartaric acid; D-ribose; riboflavin; deoxy-ribonucleotides; ribonucleotides; sorbitol; glycerol; sorbose; dihydroxyacetone; aromatic amino acids; aromatic compounds, such as P-hydroxybenzoic acids, quinones, catechol, indole, indigo, gallic acid, pyrogallol, melanin, adipic acid, and P-aminobenzoic acid; pyridoxine and aspartame. Particularly preferred desired products are keto derivatives of sorbitol, keto derivatives of gluconate and keto derivatives of glycerol.

As used herein, the term “bacteria” refers to any group of microscopic organisms that are prokaryotic, i.e., that lack a membrane-bound nucleus and organelles. All bacteria are surrounded by a lipid membrane that regulates the flow of materials in and out of the cell. A rigid cell wall completely surrounds the bacterium and lies outside the membrane. There are many different types of bacteria, some of which include, but are not limited to Bacillus, Streptomyces, Pseudomonas, and strains within the families of Enterobacteriaceae.

As used herein, the family “Enterobacteriaceae” refers to bacterial strains having the general characteristics of being gram negative and being facultatively anaerobic. For the production of ASA intermediates, preferred Enterobacteriaceae strains are those that are able to produce 2,5-diketo-D-gluconic acid from D-glucose or carbon sources which can be converted to D-glucose by the strain. (Kageyama et al., International J. Sys. Bacteriol. 42:203 (1992)). Included in the family of Enterobacteriaceae are Erwinia, Enterobacter, Gluconobacter, Klebsiella, Escherichia and Pantoea. In the present invention, a preferred Enterobacteriaceae fermentation strain for the production of ASA intermediates is a Pantoea species. The genus Pantoea includes P. agglomerans, P. dispersa, P. punctata, P. citrea, P. terrea, P. ananas and P. stewartii and in particular, Pantoea citrea. Pantoea citrea can be obtained from ATCC® (Manassas, Va.) for example ATCC® No. 39140. Pantoea citrea has sometimes been referred to as Erwinia herbicola or Acetobacter cerenius. Thus, it is intended that the genus Pantoea include species that have been reclassified, including but not limited to Erwinia herbicola or Acetobacter cerenius.

As used herein the family “Bacillus” refers to rod-shaped bacterial strains having the general characteristics of being gram positive and capable of producing resistant endospores in the presence of oxygen. Examples of Bacillus include B. subtilis, B. licheniformis, B. lentus, B. circulans, B. lautus, B. amyloliquefaciens, B. stearothermophilus, B. alkalophilus, B. coagulans, B. thuringiensis and B. brevis.

As used herein, the term “recombinant” refers to a host cell that has a modification of its genome, e.g., as by the addition of nucleic acid not naturally occurring in the organism or by a modification of nucleic acid naturally occurring in the host cell.

The term “heterologous” as used herein refers to nucleic acid or amino acid sequences not naturally occurring in the host cell. As used herein, the term “endogenous” refers to a nucleic acid or encoded amino acid naturally occurring in the host.

The terms “isolated” or “purified” as used herein refer to an enzyme, nucleic acid, protein, peptide or co-factor that is removed from at least one component with which it is naturally associated. In the present invention, an isolated nucleic acid can include a vector comprising the nucleic acid.

As used herein, the term “vector” refers to a polynucleotide construct designed to introduce nucleic acids into one or more cell types. Vectors include cloning vectors, expression vectors, shuttle vectors, plasmids and the like.

The terms “polynucleotide” and “nucleic acid”, used interchangeably herein, refer to a polymeric form of nucleotides of any length, either ribonucleotides or deoxyribonucleotides. These terms include a single-, double- or triple-stranded DNA, genomic DNA, cDNA, RNA, DNA-RNA hybrid, or a polymer comprising purine and pyrimidine bases, or other natural, chemically, biochemically modified, non-natural or derivatized nucleotide bases. The following are non-limiting examples of polynucleotides: a gene or gene fragment, exons, introns, mRNA, tRNA, rRNA, ribozymes, cDNA, recombinant polynucleotides, branched polynucleotides, plasmids, vectors, isolated DNA of any sequence, isolated RNA of any sequence, nucleic acid probes, and primers. A polynucleotide may comprise modified nucleotides, such as methylated nucleotides and nucleotide analogs, uracyl, other sugars and linking groups such as fluororibose and thioate, and nucleotide branches. The sequence of nucleotides may be interrupted by non-nucleotide components.

A polynucleotide or polypeptide having a certain percentage (for example, 40%, 50%, 60%, 70%, 80%, 85%, 90%, 93%, 95%, 97% or 99%) of “sequence identity” to another sequence means that, when aligned, that percentage of bases or amino acids are the same in comparing the two sequences. This alignment and the percent homology or sequence identity can be determined using software programs known in the art, for example those described in CURRENT PROTOCOLS IN MOLECULAR BIOLOGY (F. M. Ausubel et al., eds., 1987) Supplement 30, section 7.7.18. A preferred alignment program is ALIGN Plus (Scientific and Educational Software, Pennsylvania), preferably using default parameters, which are as follows: mismatch=2; open gap=0; extend gap=2. Another sequence software program that could be used is the TFastA Data Searching Program available in the Sequence Analysis Software Package Version 6.0 (Genetic Computer Group, University of Wisconsin, Madison, Wis.).

It is well understood in the art that the acidic derivatives of saccharides, may exist in a variety of ionization states depending upon their surrounding media, if in solution, or out of solution from which they are prepared if in solid form. The use of a term, such as, for example, idonic acid, to designate such molecules is intended to include all ionization states of the organic molecule referred to. Thus, for example, “idonic acid”, its cyclized form “idonolactone”, and “idonate” refer to the same organic moiety, and are not intended to specify particular ionization states or chemical forms.

The term “culturing” as used herein refers to fermentative bioconversion of a carbon substrate to the desired end-product within a reactor vessel. Bioconversion means contacting a microorganism with a carbon substrate to convert the carbon substrate to the desired end-product.

As used herein “ATCC®” refers to American Type Culture Collection located in Manassas, Va. 20108 (ATCC®, www/atcc.org).

As used herein, the term “comprising” and its cognates are used in their inclusive sense; that is, equivalent to the term “including” and its corresponding cognates.

“A,” “an” and “the” include plural references unless the context clearly dictates otherwise.

PREFERRED EMBODIMENTS A. Genes, Proteins and Vectors

The present application concerns isolated nucleic acid molecules encoding gluconate transporter proteins. In one embodiment, an isolated polynucleotide which encodes a gluconate transporter has the nucleic acid sequence set forth in SEQ ID NO: 1 or a nucleic acid sequence having at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 98% and 99% sequence identity to the sequence of SEQ ID NO: 1. Preferably the isolated polynucleotide will have at least 40% sequence identity, at least 80% sequence identity or at least 95% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 1. Preferably the isolated polynucleotide is obtained from a Pantoea host cell.

In another embodiment, the isolated polynucleotide which encodes a gluconate transporter has the nucleic acid sequence set forth in SEQ ID NO: 3 or a nucleic acid sequence having at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 98% and 99% sequence identity to the sequence of SEQ ID NO: 3. Preferably, the isolated polynucleotide will have at least 40% sequence identity, at least 80% sequence identity, or at least 95% sequence identity to the nucleic acid sequence set forth in SEQ ID NO: 3. Preferably, the isolated polynucleotide is obtained from a Pantoea host cell.

An isolated polynucleotide according to the invention will encode a gluconate transporter protein having the amino acid sequence set forth in SEQ ID NO: 2 or a gluconate transporter protein having at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 98% and 99% sequence identity to the sequence of SEQ ID NO: 2. Preferably the isolated polynucleotide encodes a gluconate transporter protein having the sequence as set forth in SEQ ID NO: 2 or encodes a gluconate transporter protein having at least 40% sequence identity, at least 80% sequence identity or at least 95% sequence identity to SEQ ID NO: 2. The gluconate transporter protein having the amino acid sequence as set forth in SEQ ID NO: 2 is designated herein as GntU.

An isolated polynucleotide according to the invention may also encode a gluconate transporter protein having the amino acid sequence set forth in SEQ ID NO: 4 or a gluconate transporter protein having 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 98% and 99% sequence identity to the sequence of SEQ ID NO: 4. Preferably, the isolated polynucleotide encodes a gluconate transporter protein having the sequence as set forth in SEQ ID NO: 4 or encodes a transporter protein having at least 80% sequence identity or at least 95% sequence identity to SEQ ID NO: 4. The protein having the amino acid sequence as set forth in SEQ ID NO: 4 is designated herein as IdnT. Additionally, in one embodiment this gluconate transporter and those having at least 80% sequence identity thereto are also able to facilitate the transport of idonate across the inner cellular membrane of a host bacterial cell.

One of skill in the art is well aware of the degeneracy of the genetic code and that an amino acid may be coded for by more than one codon. These variations are include as part of the invention herein.

A further embodiment of the invention includes an isolated gluconate transporter protein comprising the amino acid sequence set forth in SEQ ID NO: 2 or an amino acid sequence having at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 98% and 99% sequence identity to the sequence of SEQ ID NO: 2. In one embodiment, the gluconate transporter will have the amino acid sequence set forth in SEQ ID NO: 2 or at least 40%, at least 80% or at least 95% sequence identity thereof. The gluconate transporter protein set forth in SEQ ID NO: 2 was obtained from Pantoea. In another embodiment, the invention includes an isolated gluconate transporter protein comprising the amino acid sequence set forth in SEQ ID NO: 4 or an amino acid sequence having at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 93%, 95%, 97%, 98% and 99% sequence identity to the sequence of SEQ ID NO: 4. In one embodiment, the gluconate transporter will have the amino acid sequence set forth in SEQ ID NO: 4 or at least 80% sequence thereto. In this embodiment the gluconate transporter will also facilitate the transport of idonate. The gluconate transporter protein set forth in SEQ ID NO: 4 was obtained from Pantoea. In one embodiment, a preferred gluconate transporter is a transporter obtained from bacterial strains of Pantoea, particularly a strain of P. citrea or P. agglomerans.

A gluconate transporter according to the invention will catalyze the transport of gluconate but also may catalyze the transport of the sugar-keto acids, KDG, KLG, 5-KDG, D-Tagaturonic acid, and D-Fructuronic acid. Further a gluconate transporter according to the invention may also catalyze the transport of other sugar acids, such as idonic acid, galacturonic acid, gulonic acid and/or glucuronic acid.

Gluconate transporters according to the invention are members of the GntP family of transporters. Over 100 gluconate transporter proteins from various microorganisms have been identified. These microorganisms include Escherichia, Bacillus, Haemophilus, Pseudimonas and Clostridium. (Reference is made to Yamada et al., (1996) Biosci. Biotech. Biochem. 60:1548-1550; Bausch et al., (1998) J. Bacteriol. 180:3701-3710; Reizer et al., (1991) Mol. Microbiol. 5:1081-1089 and Peekhaus et al., (1997) FEMS Microbiol. Lett. 147:233-238). The overall identity of some of the disclosed gluconate transporter proteins with the amino acid sequence illustrated in SEQ ID NO: 2 is less than about 34% identity and for SEQ ID NO: 4 less than about 76.3% identity (FIG. 10).

Naturally occurring gluconate transporter proteins have about 12 to 14 transmembrane domains and typically are about 400 to 450 amino acids in length. In this embodiment, a preferred isolated gluconate transporter is a protein obtained from bacterial strains of Pantoea, particularly a strain of P. citrea or P. agglomerans.

Methods useful for identifying gluconate transporter proteins found in bacterial microorganisms such as Pantoea species are well known in the art and would include hybridization studies. Thus, for example, nucleic acid sequences which hybridize under high stringency conditions to a gluconate transporter gene identified in FIGS. 4 and 6 or a complement thereof, may also encode proteins that function as a gluconate transporter. Hybridization includes the process by which a strand of nucleic acid joins with a complementary strand through base pairing. High stringency conditions are known in the art and see for example, Maniatis, et al., MOLECULAR CLONING: A LABORATORY MANUAL, 2d Edition (1989) and SHORT PROTOCOLS IN MOLECULAR BIOLOGY, ed Ausubel et al. Stringent conditions are sequence dependent and will be different in different circumstances. Longer sequences hybridize specifically at higher temperatures. An extensive guide to the hybridization of nucleic acids is found in Tijssen, TECHNIQUES IN BIOCHEMISTRY AND MOLECULAR BIOLOGY—HYBRIDIZATION WITH NUCLEIC ACID PROBES, Overview of Principles of Hybridization and the Strategy of Nucleic Acid Assays (1993). Generally stringent conditions are selected to be about 5 to 10 degrees lower than the thermal melting point Tm for the specific sequence at a defined ionic strength and pH. The Tm is the temperature (under defined ionic strength, pH and nucleic acid concentration) at which 50% of the probes complementary to the target, hybridize to the target sequence at equilibrium. Stringent conditions will be those in which the salt concentration is less than about 1.0 M sodium ion, typically about 0.01 to 1.0M sodium ion concentration (or other salts) at pH 7.0 to 8.3 and the temperature is at least 30° C. for short probes (e.g. for 10 to 50 nucleotides) and at least about 60° C. for long probes (e.g. greater than 50 nucleotide). Stringent conditions may also be achieved with the addition of destabilizing agents such as formamide. In another embodiment, less stringent hybridization conditions are used. For example moderate or low stringent conditions may be used.

PCR may also be used to screen for homologous sequences of gluconate transporters and reference is made to Chen et al., (1995) Biotechniques 18(4):609-612. Exemplary probe oligonucleotides that may be used for detecting gluconate transporter molecules include the polynucleotides set forth in SEQ ID NO:1 or SEQ ID NO: 3 or fragments thereof. These fragments may include at least 15, 20, 25, 30, 35, 40, 45, 50, 60, 65, 70, 75, 80, 85, 90 or more contiguous nucleotides from the reference sequence of SEQ ID NO: 1 or SEQ ID NO: 3. Additionally, an oligonucleotide useful in the present invention may comprise a nucleotide sequence encoding a polypeptide having at least 5, 10, 15, 20, 25, 30 or more contiguous amino acid residues of either SEQ ID NO: 2 or SEQ ID NO: 4. Other methods include protein bioassay or immunoassay techniques which include membrane-based, solution-based, or chip-based technologies for the detection and/or quantification of the nucleic acid or protein.

Gluconate transporter activity can be determined by a variety of methods. For example, using gluconate uptake assays and detection of metabolic products of gluconate or idonate such as by using HPLC as described further in the examples.

Moreover, it is believed that within the cells of the same species or strain, more than one gluconate transporter protein may be involved in the transport of gluconate into the cytoplasm. In one example, a family of gluconate transporter proteins of which GntU and IdnT are members would comprise a gluconate transporter system in a Pantoea cell.

Transporter proteins function by moving substrates across a cellular membrane. As shown in FIG. 2, the function of a gluconate transporter is to facilitate transport of a substrate, such as gluconate into the cytoplasm where it is then made available for cellular metabolism. By reducing the transport of gluconate into the cytoplasm by rendering a gluconate transporter gene non-functional, more substrate will be available for oxidative production of keto-sugars and ASA intermediates and particularly for KLG bioproduction.

B. Bacterial Host Cells

Particularly preferred bacterial host cells according to the invention are Enterobacteriaceae cells and particularly E. coli and Pantoea cells. Particularly preferred Pantoea cells are P. citrea and P. aggolmerans (U.S. Pat. No. 5,032,514). Bacillus sp. may also serve as host cells.

In one embodiment, an altered bacterial cell of the invention comprises at least one non-functional endogenous gluconate transporter. In one example, the altered bacterial cell would include a non-functional gluconate transporter wherein the gluconate transporter in the corresponding unaltered bacterial cell was encoded by a polynucleotide encoding the amino acid sequence set forth in SEQ ID NO: 2 or a polynucleotide encoding a gluconate transporter having an amino acid sequence of at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97%, 98% or 99% identity with the sequence of SEQ ID NO: 2. In a further embodiment, the gluconate transporter in the unaltered bacterial host cell will be encode by a polynucleotide having the nucleic acid sequence of at least 80%, 85%, 90%, 95%, 97%, 98% and 99% identity with the sequence of SEQ ID NO: 1.

In another example, the altered bacterial cell would include a non-functional gluconate transporter wherein the gluconate transporter in the corresponding unaltered bacterial cell was encoded by a polynucleotide encoding the amino acid sequence set forth in SEQ ID NO: 4 or a polynucleotide encoding a gluconate transporter having an amino acid sequence with at least 80%, 85%, 90%, 95%, 97%, 98% or 99% identity with the sequence of SEQ ID NO: 4. In a further embodiment, the gluconate transporter in the unaltered bacterial host cell will be encoded by a polynucleotide having the nucleic acid sequence of at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97% and 99% identity with the sequence of SEQ ID NO: 3.

Altered bacterial cells according to the invention may include more than one non-functional gluconate transporter protein. Altered cells according to the invention may include two, three, four or more non-functional gluconate transporter proteins. In some embodiments, at least two gluconate transporter proteins are rendered non-functional. In one specific example, a gluconate transporter having at least 90% sequence identity with the sequence SEQ ID NO: 2 and a gluconate transporter having at least 90% sequence identity with the sequence of SEQ ID NO: 4 are rendered non-functional in a bacterial host cell. In a second specific example, a gluconate transporter having at least 95% sequence identity with the sequence SEQ ID NO: 2 and a gluconate transporter having at least 95% sequence identity with the sequence of SEQ ID NO: 4 are rendered non-functional in a bacterial host cell

If a gluconate transporter is rendered non-functional, the transport of the substrate gluconate across the inner cell membrane will be substantially reduced and in some cases gluconate will not be transported across the inner cell membrane. The reduction of gluconate transport results in increased availability of the gluconate substrate in the oxidative pathway for use in production of ASA intermediates.

C. Methods for Rendering Gluconate Transporters Non-Functional

In general, methods of rendering chromosomal genes non-functional are well known and a number of these techniques may be used to inactivate a gluconate transporter molecule according to the invention. In an altered bacterial strain according to the invention, the inactivation of the gluconate transporter genes will preferably be a stable and non-reverting inactivation. Some of these methods include the integration of DNA into a host cell and reference is made to Balbas et al., (1996), Gene, 172:65-69 and LeBorge et al., (1998), Gene, 223:213-219.

One preferred method of inactivation is a deletion of the gene encoding the gluconate transporter. The deletion may be partial as long as the sequences left in the chromosome are too short for biological activity of the gene. One method of deleting a gluconate transporter gene according to the invention includes constructing a vector which includes homologous flanking regions of the transporter coding sequence of interest. The flanking regions may include from about 1 bp to about 500 bp at the 5′ and 3′ ends. The flanking region of the vector may be larger than 500 bp but will preferably not include other genes in the region of the gene to be deleted which would result in these genes being inactivated or deleted. The vector construct may be introduced into a host cell by, for example, transformation and then integrated into the host cell chromosome. The end result is that the introduced DNA causes the endogenous gluconate gene to be deleted.

In another embodiment, inactivation is by insertion. Insertional inactivation includes interruption of the chromosomal coding region. For example when gntU is the gene to be inactivated, a DNA construct will comprise a nucleic acid sequence having the gntU gene interrupted by a selective marker. The selective marker will be flanked on each side by sections of the gntU coding sequence. The DNA construct aligns with essentially identical sequences of the gntU gene in the host chromosome and in a double crossover event the gntU gene is inactivated by the insertion of the selective marker. A selectable marker refers to a gene capable of expression in the host microorganism which allows for ease of selection of those hosts containing the vector. Examples of such selectable markers include but are not limited to antibiotic resistant genes such as, erythromycin, kanamycin, chloramphenicol and tetracycline.

In another embodiment, inactivation is by insertion in a single crossover event with a plasmid as the vector. For example, a gluconate transporter chromosomal gene such as gntU is aligned with a plasmid comprising the gntU gene or part of the gene coding sequence and a selective marker. The selective marker may be located within the gene coding sequence or on a part of the plasmid separate from the gene. The vector may be integrated into the bacterial chromosome, and the gene is inactivated by the insertion of the vector in the coding sequence.

Inactivation may also occur by a mutation of the gene. Methods of mutating genes are well known in the art and include but are not limited to chemical mutagenesis, site-directed mutation, generation of random mutations, and gapped-duplex approaches. (U.S. Pat. No. 4,760,025; Moring et al., Biotech. 2:646 (1984); and Kramer et al., Nucleic Acids Res. 12:9441 (1984)). Chemical mutagenesis may include the use chemicals that affect nonreplicating DNA, such as HNO₂ and NH₂OH, as well as agents that affect replicating DNA, such as acridine dyes. Specific methods for creating mutants using radiation and chemical agents are well documented in the art, see, for example T. D. Brock in BIOTECHNOLOGY: A TEXTBOOK OF INDUSTRIAL MICROBIOLOGY, 2nd Ed. (1989) Sinauer Associates, Inc. Sunderland, Mass. After mutagenesis has occurred, mutants having the desired property may be selected by a variety of methods, such as random screening or selective isolation on a selective media.

In some embodiments, an expression vector is constructed which contains a multiple cloning site cassette which preferably comprises at least one restriction endonuclease site unique to the vector, to facilitate ease of nucleic acid manipulation. In a preferred embodiment, the vector also comprises one or more selectable markers. In another preferred embodiment, a gluconate transporter gene is deleted by homologous recombination.

For example, as shown in FIG. 7, an inactivation cassette is constructed by first cloning a DNA fragment containing the gluconate transporter gene, gntU into a vector. To inactivate the gntU gene, an antibiotic resistance gene (i.e. a chloramphenicol, Cm^(R) gene) is cloned into a unique restriction site found in the gluconate transporter gene. The insertion of the antibiotic marker into the gntU gene interrupts its normal coding sequence. The inactivation cassette is transferred to the host cell chromosome by homologous recombination using a non-replication R6K vector like pGP704 (Miller et al. (1988) J. Bacteriol. 170:2575-2583). The transfer of the cassette into the host cell chromosome is selected by the inclusion of Cm^(R) by the host cell. Once the inactivation of the gntU gene has been corroborated, the Cm^(R) is removed from the gntU coding region leaving an interrupting spacer (which in this example includes a copy of a loxP site) in the coding region, inactivating the coding region. (Palmeros et al., (2000) Gene 247:255-264). A similar process is seen in FIG. 8 for inactivation of the endogenous idnT gene. However in this example two fragments, one including a loxP-Cm^(R)-idnT cassette and one including a idnD cassette are ligated together and transformed into the host resulting in both the interruption of the gluconate transporter, idnT and the gene idnD which encodes idonate dehydrogenase.

Plasmids which can be used as vectors in bacterial organisms are well known and reference is made to Maniatis, et al., MOLECULAR CLONING: A LABORATORY MANUAL, 2d Edition (1989) AND MOLECULAR CLONING: A LABORATORY MANUAL, second edition (Sambrook et al., 1989) and Bron, S, Chapter 3, Plasmids, in MOLECULAR BIOLOGY METHODS FOR BACILLUS, Ed. Harwood and Cutting, (1990) John Wiley & Sons Ltd.

A preferred plasmid for the recombinant introduction of polynucleotides encoding non-naturally occurring proteins or enzymes into a strain of Enterobacteriaceae is RSF1010, a mobilizable, but not self transmissible plasmid which has the capability to replicate in a broad range of bacterial hosts, including Gram− and Gram+ bacteria. (Frey et al., 1989, The Molecular Biology of IncQ Plasmids. In: Thomas (Ed.), PROMISCUOUS PLASMIDS OF GRAM NEGATIVE BACTERIA. Academic Press, London, pp. 79-94). Frey et al. report on three regions found to affect the mobilization properties of RSF1010 (Frey et al. (1992) Gene 113:101-106).

In a further embodiment, the expression product of an inactivated or deleted gene may be a truncated protein as long as the protein has a change in its biological activity. The change in biological activity could be an altered activity, but preferably is loss of biological activity.

D. Additional Gene Modifications

In one embodiment, altered bacterial cells according to the invention may include further modifications of chromosomal genes. These modifications to a bacterial host cell may have been realized prior to, simultaneously with, or after inactivation of one or more gluconate transporter genes. The chromosomal modifications may include inactivations, such as deletions or interruptions of endogenous chromosomal genes, modifications resulting in increased expression of endogenous chromosomal genes, and inclusion of heterologous genes. More specific examples of modifications to altered host cells of the invention include but are not limited to:

-   -   (i) modifications concerning additional gluconate transporter         genes;     -   (ii) modifications to genes encoding glucose transporters;     -   (iii) modifications to genes encoding KDG transporters;     -   (iv) modifications to genes encoding DKG transporters (DKG         permease genes), such as prmA, prmB, PE6, PE1 and YiaX2 (See WO         02/12468; WO 02/12481 and WO 02/12528);     -   (v) gene modifications resulting in overexpression of certain         genes, for example overexpression of DKG permease genes or aroZ         genes, which encode dehydratases; and     -   (vi) modifications to a polynucleotide that uncouples the         catabolic pathway from the oxidative pathway such as by         inactivating an enzyme that phosphorylates D-glucose or         D-gluconate at its 6th carbon and more specifically inactivating         a hexokinase gene, a glucokinase gene or a gluconokinase gene         i.e. gntK and/or glkA and reference is made to WO 02/081440.

In certain preferred embodiments, an altered bacterial strain according to the invention will include an inactivated gntK and/or an inactivated glkA gene. In another preferred embodiment, an altered bacterial strain according to the invention will include an overexpressed DKG permease.

In certain embodiments, the altered bacterial strain may embody two inactivated genes, three inactivated genes, four inactivated genes, five inactivated genes, six inactivated genes or more. The inactivated genes may be contiguous to one another or may be located in separate regions of the bacterial chromosome. An inactivated chromosomal gene may have a necessary function under certain conditions, but the gene is not necessary for bacterial strain viability under laboratory conditions. Preferred laboratory conditions include but are not limited to conditions such as growth in a fermentator, in a shake flask, in plate media or the like.

In some embodiments, the altered bacterial host cells are recombinant Pantoea cells which comprise a first inactivated endogenous gluconate transporter gene, which prior to inactivation encoded a gluconate transporter having at least 80%, 90%, and 95% sequence identity with SEQ ID NO: 2, a second inactivated endogenous gluconate transporter gene, which prior to inactivation encoded a gluconate transporter having at least 80%, 90% and 95% sequence identity with SEQ ID NO; 4, and an inactivated endogenous glucokinase gene. In some embodiments, the inactivated gluconate transporter genes will have been deleted from the altered host cell.

In another embodiment, the host cell may be genetically engineered to include genes encoding enzymes known to effect the conversion of glucose or other ordinary metabolites to 2,5-DKG or 2-KLG from the organisms known to contain them. Examples of the enzymes effecting the conversion of an ordinary metabolite to 2,5-DKG or 2-KLG are D-glucose dehydrogenase (Adachi, O. et al., (1980) Agric. Chem., 44:301-308; Ameyama, M. et al., (1981) Agric. Biol. Chem. 45:851-861; Smith et al. (1989) Biochem. J. 261:973; and Neijssel et al., (1989) Antonie Van Leauvenhoek 56(1):51-61); D-gluconate dehydrogenase (McIntire, W. et al., (1985) Biochem. J., 231:651-654; Shinagawa, E. et al., (1976) Agric. Biol. Chem. 40:475-483; Shinagawa, E. et al., (1978) Agric. Biol. Chem. 42:1055-1057; and Matsushita et al. (1979), J. Biochem. 85:1173); 5-keto-D-gluconate dehydrogenase and 2-keto-D-gluconate dehydrogenase (Shinagawa, E. et al., (1981) Agric. Biol. Chem., 45:1079-1085 and Stroshane (1977) Biotechnol. BioEng. 19(4) 459); and 2,5-diketo-D-gluconic acid reductase (U.S. Pat. Nos. 5,795,761; 5,376,544; 5,583,025; 4,757,012; 4,758,514; 5,008,193; 5,004,690; and 5,032,514).

A preferred altered bacterial strain according to the invention will be an E. coli, Bacillus or Pantoea strain and particularly a Pantoea strain such as a P. citrea. Preferably, the altered bacterial strain will comprise an inactivated endogenous gluconate transporter gene and one or more of the following: a) an inactivated endogenous glucokinase gene; b) an inactivated endogenous gluconokinase gene; c) an inactivated endogenous glycerol kinase gene; d) an over-expressed DKG transporter gene and e) an inactivated or disrupted glucose transport system, such as the phosphoenolpyruvate glucose phosphotransferase system (PTS).

In one embodiment when KLG is the desired intermediate, nucleic acid encoding 2,5 DKG reductase (DKGR) is recombinantly introduced into the bacterial fermentation strain. Many species have been found to contain DKGR particularly members of the Coryneform group, including the genera Corynebacterium, Brevibacterium, and Arthrobacter. In a further embodiment DKGR from Corynebacterium sp. strain SHS752001 (Grindley et al., 1988, Applied and Environmental Microbiology 54: 1770-1775) is recombinantly introduced into a host strain, such as a Pantoea host strain. Production of recombinant 2,5 DKG reductase by Erwinia herbicola is disclosed in U.S. Pat. No. 5,008,193.

Gene transfer techniques for bacterial cells are well known and these techniques include transformation, transduction, conjugation and protoplast fusion. Gene transfer is the process of transferring a gene or polynucleotide to a cell or cells wherein exogenously added DNA is taken up by a bacterium. General transformation procedures are taught in CURRENT PROTOCOLS IN MOLECULAR BIOLOGY (vol. 1, edited by Ausubel et al., John Wiley & Sons, Inc. 1987, Chapter 9) and include calcium phosphate methods, transformation using DEAE-Dextran and electroporation. A variety of transformation procedures are known by those of skill in the art for introducing nucleic acids in a given host cell. (Reference is also made to U.S. Pat. No. 5,032,514; Potter H. (1988) Anal. Biochem 174:361-373; Sambrook, J. et al., MOLECULAR CLONING: A LABORATORY MANUAL, Cold Spring Harbor Laboratory Press (1989); and Ferrari et al., Genetics, pgs 57-72 in BACILLUS, Harwood et al., Eds. Plenum Publishing Corp).

Transformation of a host cell can be detected by the presence/absence of marker gene expression which can suggest whether the nucleic acid of interest is present. However, its expression should be confirmed. For example, if the nucleic acid encoding a gluconate transporter is inserted within a marker gene sequence, recombinant cells containing the insert can be identified by the absence of marker gene function. Alternatively, a marker gene can be placed in tandem with nucleic acid encoding the transporter under the control of a single promoter. Expression of the marker gene in response to induction or selection usually indicates expression of the protein or enzyme as well. Once a bacterial microorganism capable of carrying out the conversion as described above has been created, the process of the invention may be carried out in a variety of ways depending on the nature of the construction of the expression vectors for the recombinant proteins and upon the growth characteristics of the host.

E. Cell Cultures and Fermentations

Methods suitable for the maintenance and growth of bacterial cells is well known and reference is made to the MANUAL OF METHODS OF GENERAL BACTERIOLOGY, Eds. P. Gerhardt et al., American Society for Microbiology, Washington D.C. (1981) and T. D. Brock in BIOTECHNOLOGY: A TEXTBOOK OF INDUSTRIAL MICROBIOLOGY, 2nd ed. (1989) Sinauer Associates, Sunderland, Mass.

Cell Precultures—Typically cell cultures are grown at 25 to 32° C., and preferably about 28 or 29° C. in appropriate media. Exemplary growth media useful in the present invention are common commercially prepared media such as but not limited to Luria Bertani (LB) broth, Sabouraud Dextrose (SD) broth or Yeast medium (YM) broth. These may be obtained from for example, GIBCO/BRL (Gaithersburg, Md.). Other defined or synthetic growth media may be used and the appropriate medium for growth of the particular bacterial microorganism will be known by one skilled in the art of microbiology or fermentation science. Suitable pH ranges preferred for the fermentation are between pH 5 to pH 8. Preferred ranges for seed flasks are pH 7 to pH 7.5 and preferred ranges for the reactor vessels are pH 5 to pH 6. It will be appreciated by one of skill in the art of fermentation microbiology that a number of factors affecting the fermentation processes may have to be optimized and controlled in order to maximize the ascorbic acid intermediate production. Many of these factors such as pH, carbon source concentration, and dissolved oxygen levels may affect enzymatic processes depending on the cell types used for ascorbic acid intermediate production.

The production of ASA intermediates can proceed in a fermentative environment, that is, in an in vivo environment, or a non-fermentative environment, that is, in an in vitro environment; or combined in vivo/in vitro environments. The fermentation or bioreactor may be performed in a batch process, a Fed-batch process or in a continuous process.

In Vivo Biocatalytic Environment

Biocatalysis begins with culturing an altered host cell according to the invention in an environment with a suitable carbon source ordinarily used by Enterobacteriaceae or other bacterial strains. Suitable carbon sources include 6 carbon sugars, for example, glucose, or a 6 carbon sugar acid, or combinations of 6 carbon sugars and/or 6 carbon sugar acids. Other carbon sources include, but are not limited to galactose, lactose, fructose, or the enzymatic derivatives of such.

In addition, fermentation media must contain suitable carbon substrates which will include but are not limited to monosaccharides such as glucose, oligosaccharides such as lactose or sucrose, polysaccharides such as starch or cellulose and unpurified mixtures from a renewable feedstocks such as cheese whey permeate, cornsteep liquor, sugar beet molasses, and barley malt. Additionally, the carbon substrate may be one-carbon substrates such as carbon. While it is contemplated that the source of carbon utilized in the present invention may encompass a wide variety of carbon containing substrates and will only be limited by the choice of organism, the preferred carbon substrates include glucose, fructose and sucrose and mixtures thereof. By using mixtures of glucose and fructose in combination with the altered bacterial strains described herein, uncoupling of the oxidative pathways from the catabolic pathways allows the use of glucose for improved yield and conversion to the desired ASA intermediate while utilizing the fructose to satisfy the metabolic requirements of the host cells.

In one embodiment, the concentration of the carbon substrate in the seed solution is from about 55% to about 75% on a weight/weight basis. Preferably, the concentration is from about 60 to about 70% on a weight/weight basis. Most preferably the concentration used is 60% to 67% glucose.

In addition to an appropriate carbon source, fermentation media must contain suitable minerals, salts, vitamins, cofactors and buffers suitable for the growth or the cultures and promotion of the enzymatic pathway necessary for ascorbic acid intermediate production.

Batch, Fed-Batch and Continuous Fermentations:

The present invention may employ a batch fermentation process, a modified batch fermentation process called Fed-batch or a continuous fermentation process.

A classical batch fermentation is a closed system where the composition of the media is set at the beginning of the fermentation and not subject to artificial alterations during the fermentation. At the beginning of the fermentation the media is inoculated with the desired bacterial organism or organisms and fermentation is permitted to occur adding nothing to the system. Typically, however, a “batch” fermentation is batch with respect to the addition of carbon source and attempts are often made at controlling factors such as pH and oxygen concentration. In batch systems the metabolite and biomass compositions of the system change constantly up to the time the fermentation is stopped. Within batch cultures, cells moderate through a static lag phase to a high growth log phase and finally to a stationary phase where growth rate is diminished or halted. If untreated, cells in the stationary phase will eventually die. Cells in log phase generally are responsible for the bulk of production of desired end products or intermediates.

A variation on the standard batch system is the Fed-Batch system. Fed-Batch fermentation processes are also suitable in the present invention and comprise a typical batch system with the exception that the substrate is added in increments as the fermentation progresses. Fed-Batch systems are useful when catabolite repression is apt to inhibit the metabolism of the cells and where it is desirable to have limited amounts of substrate in the media. Measurement of the actual substrate concentration in Fed-Batch systems is difficult and is therefore estimated on the basis of the changes of measurable factors such as pH, dissolved oxygen and the partial pressure of waste gases such as CO₂. Batch and Fed-Batch fermentations are common and well known in the art and examples may be found in T. D. Brock in BIOTECHNOLOGY: A TEXTBOOK OF INDUSTRIAL MICROBIOLOGY, Second Edition (1989) Sinauer Associates, Inc. Sunderland, Mass.

Continuous fermentation is an open system where a defined fermentation media is added continuously to a bioreactor and simultaneously an equal amount of conditioned media is removed for processing. Continuous fermentation generally maintains the cultures at a constant high density where cells are primarily in log phase growth. Continuous fermentation allows for the modulation of one factor or any number of factors that affect cell growth or end product concentration. For example, one method will maintain a limiting nutrient such as the carbon source or nitrogen level at a fixed rate and allow all other parameters to moderate. In other systems a number of factors affecting growth can be altered continuously while the cell concentration, measured by media turbidity, is kept constant. Continuous systems strive to maintain steady state growth conditions and thus the cell loss due to media being drawn off must be balanced against the cell growth rate in the fermentation. Methods of modulating nutrients and growth factors for continuous fermentation processes as well as techniques for maximizing the rate of product formation are well known in the art of industrial microbiology and a variety of methods are detailed by Brock, supra.

In one illustrative in vivo example of a pathway in Pantoea, D-glucose undergoes a series of membrane oxidative steps through enzymatic conversions, which may include the enzymes D-glucose dehydrogenase, D-gluconate dehydrogenase and 2-keto-D-gluconate dehydrogenase to give intermediates which may include, but are not limited to GA, KDG, and DKG, see FIGS. 1 and 2. These intermediates undergo a series of intracellular reducing steps through enzymatic conversions, which may include the enzymes 2,5-diketo-D-gluconate reductase (DKGR), 2-keto reductase (2-KR) and 5-keto reductase (5-KR) to give end products which include but are not limited to KLG and IA. In a preferred embodiment of the in vivo environment for the production of ASA intermediates, gluconate is not transported across the host cell membrane which results in an increased availability of gluconate for oxidative pathway production of ASA intermediates.

F. Recovery, Identification and Purification of ASA Intermediates

Methods for detection of ASA intermediates, ASA and ASA stereoisomers such as D-araboascorbic acid (erythorbic acid), L-araboascorbic acid, and D-xyloascorbic acid include the use of redox-titration with 2,6 dichloroindophenol (Burton et al. (1979) J. Assoc. Pub. Analysts 17:105) or other suitable reagents; high-performance liquid chromatography (HPLC) using anion exchange; and electro-redox procedures (Pachia, (1976) Anal. Chem. 48:364). The skilled artisan will be well aware of controls to be applied in utilizing these detection methods. Alternatively, the intermediates can also be formulated directly from the fermentation broth or bioreactor and granulated or put in a liquid formulation. KLG produced by a process of the present invention may be further converted to ascorbic acid and the KDG to erythorbate by means known to those of skill in the art, see for example, Reichstein and Grussner, Helv. Chim. Acta., 17, 311-328 (1934).

G. Increased Yield of Desired Products from the ASA Pathway

The catabolic pathway is uncoupled from the ASA oxidative pathway to increase the availability of gluconate or idonate for ASA intermediate production. As shown in FIGS. 1 and 2 gluconate can be transported across the inner cell membrane to the cytoplasm. Gluconate may then be phosphorylated, for example to gluconate-6-phosphate and made available for catabolic metabolism in the pentose pathway. Additionally, gluconate may be enzymatically reduced to 5-KDG or 2-KDG in the cytoplasm. Inactivation or modification of the levels of a gluconate transporter according to the invention by inactivation of the nucleic acid encoding the same, results in an increased amount of gluconate available for the oxidative ASA production pathway resulting in a desired product. (donate may also be transported by the gluconate transporter across the inner membrane into the cytoplasm and through a series of enzymatic steps be converted to gluconate. This gluconate may then be phosphorylated as described above. Inactivation or modification of the levels of a gluconate transporter according to the invention by inactivation of the nucleic acid encoding the same, results in an increased amount of idonate available for conversion to 2-KLG. (FIGS. 1 and 2).

In an embodiment of the invention, the catabolic pathway is uncoupled from the ASA oxidative pathway to increase the production of KDG. Inactivation of the gluconate transporter diminishes the transport of gluconate across the inner cell membrane and reduces the availability of gluconate for phosphorylation by a kinase. Inactivation of the gluconate transporter gene or modification of the levels of the gluconate transporter results in the increased yield of a desired product, e.g. KDG.

In another embodiment, the catabolic pathway is uncoupled from the productive oxidative pathway to increase the production of DKG. Inactivation or modifying the levels of the gluconate transporter by modifying the nucleic acid or polypeptide encoding the same, results in the increased yield of the desired product, e.g. DKG.

In a further embodiment, the catabolic pathway is uncoupled from the productive oxidative pathway to increase the production of KLG. Inactivation or modifying the levels of the gluconate transporter by modifying the nucleic acid or polypeptide encoding the same, results in the increased yield of the desired product, e.g. KLG.

In another embodiment, the catabolic pathway is uncoupled from the productive pathway to increase the production of erythorbic acid. Inactivation or modifying the levels of the gluconate transporter by modifying the nucleic acid or polypeptide encoding the same, results in the increased yield of DKG which is an intermediate in the synthesis of erythorbic acid.

General Experimental Methods

Cells: All commercially available cells used in the following examples were obtained from the ATCC® and are identified in the text by their ATCC® number. Recombinant P. citrea cells used as a control were derived from ATCC 39140. (Also reference is made to Truesdell et al., (1991) J. Bacteriol. 173:6651-6656).

Materials and Methods suitable for the maintenance and growth of bacterial cultures were found in MANUAL OF METHODS FOR GENERAL BACTERIOLOGY (Phillipp Gerhardt, R. G. E. Murray, Ralph N. Costilow, Eugene W. Nester, Willis A. Wood, Noel R. Krieg and G. Briggs Phillips, eds), pp. 210-213. American Society for Microbiology, Washington, D.C. or in Thomas D. Brock in BIOTECHNOLOGY: A TEXTBOOK OF INDUSTRIAL MICROBIOLOGY, Second Edition (1989) Sinauer Associates, Inc., Sunderland, Mass. All reagents and materials used for the growth of bacterial cells were obtained from Diffco Laboratories (Detroit, Mich.), Aldrich Chemicals (Milwaukee, Wis.) or Sigma Chemical Company (St. Louis, Mo.) unless otherwise specified.

Ascorbic acid intermediate analysis: The presence of ascorbic acid intermediates, e.g., 2-KLG, was verified by running a HPLC analysis. Fermentation reactor vessel samples were drawn off the tank and loaded onto Dionex (Sunnyvale, Calif., Product No. 043118) Ion Pac AS 10 column (4 mm times 250 mm) connected to a Waters 2690 Separation Module and a Waters 410 Differential Refractometer (Milford, Mass.).

The manner and method of carrying, out the present invention may be more fully understood by those of skill in the art by reference to the following examples, which examples are not intended in any manner to limit the scope of the present invention or of the claims directed thereto. All references and patent publications referred to herein are hereby incorporated by reference.

EXAMPLES Example 1 Inactivation of the gntU ORF Encoding a Gluconate Transporter in P. citrea

Cloning of the gntKU Operon:

A 1934-bp DNA fragment containing the gntK and gntU structural genes from Pantoea citrea was amplified by PCR with GntKUF1 (SEQ ID NO: 5) plus GntKUR1 (SEQ ID NO: 6) primer pairs using standard techniques (Sambrook et al., MOLECULAR CLONING: A LABORATORY MANUAL, Cold Spring Harbor Laboratory Press (1989) 2nd Ed.). In addition to the structural genes, the PCR fragment carries 27 bp upstream of the gntK gene and 23 bp downstream of the gntU (FIG. 7). This PCR product was cloned into the HindIII site of pCL1920 vector resulting in plasmid pCL-GntKU2 (6489-bp) (Lerner et al., (1990) Nucleic Acids Res. 18:4631). This was constructed into E. coli TOP10 strain (Invitrogen, Carlsbad, Calif.) and checked by EcoRI and PstI restriction enzymes for correct insert and orientation.

Construction of the plasmid used to inactivate gntU: In general, the strategy used to inactivate genes by homologous recombination with a vector has been delineated before and reference is made to Miller et al., (1988) J. Bacteriol. 170:2575-2583. This general approach was used to inactivate gntU.

The pCL-GntKU2 plasmid described above was used for inactivation of the chromosomal gntU. pCL-GntKU2 was digested with NsiI enzyme (Roche Applied Science) to remove the 435-bp region from the middle of the gntU gene. A cat-loxP, 1080-bp cassette (Palmeros et al., (2000) Gene 247:255-264) was inserted into the construct resulting in plasmid pCLKUCat1 (7131-bp). The orientation of the cat-loxP cassette was checked with Scal1+SpeI restriction enzymes. The 2580-bp gntKU+cat-loxP region was amplified by PCR using primers GntKUF1 (SEQ ID NO: 5) and GntKUR1 (SEQ ID NO: 6), and ligated with the 502-bp DNA fragment that contains the minimal R6K origin of replication (ori). This particular DNA was obtained by PCR using plasmid pGP704 (Miller et al., (1988) J. Bacteriol. 170:2575-2583) as PCR substrate with primers

R6K1 (TGTCAGCCGTTAAGTGTTCCTGTG) (SEQ ID NO: 18) and R6K2 (CAGTTCAACCTGTTGATAGTACG). (SEQ ID NO: 19)

The ligation product was transformed into E. coli PIR1 strain (Invitrogen, Carlsbad, Calif.) to generate the plasmid pKUCatR6-1 (3087-bp). This plasmid was used to inactivate the gntU gene. In this construct, a 650-bp and 850-bp region of homology is available at the 5′- and 3′-end of the gntU gene, respectively, for homologous recombination into the P. citrea chromosome. This recombination event should not, affect the proximal gntK gene in the gntKU operon.

Transformation into P. citrea strains: After checking pKUCatR6-1 with AatII+AccI restriction enzymes, the plasmid was used to transform the strain MDP41 of P. citrea. All transformations were done by standard electroporation. (Sambrook et al., MOLECULAR CLONING: A LABORATORY MANUAL, Cold Spring Harbor Laboratory Press (1989) 2nd Ed.). Transformants were selected on LA+Cm12 plates (LA is LB plus agar+plates with 12 ppm chloramphenicol (Cm). Hundreds of Cm-resistant (CmR) colonies appeared. Four colonies were checked by PCR analysis as described below.

Verification of the altered gntU strains: To distinguish between single and double crossover events, the junction points were checked with one outside primer plus one cat gene specific primer. With GntKUF3 (outside) (SEQ ID NO: 7)+cat4 (SEQ ID NO: 11) and GntKUF4 (outside) (SEQ ID NO: 8)+cat4 (SEQ ID NO: 11) primer pairs, all four transformants amplified a 0.99-kb and a 0.95-kb fragment, respectively. With cat3 (SEQ ID NO: 10)+GntKUR3 (outside) (SEQ ID NO: 9), only No. 2 and No. 4 transformants amplified the expected 1.45-kb. Thus, No. 2 and No. 4 transformants had undergone a double cross-over event wherein the gntU was deleted. In contrast, No. 1 and No. 3 transformants had undergone a single cross-over event wherein the gntU remained intact at the gntU locus.

The 1.5-kb and 1.0-kb PCR products obtained with GntKUF4+cat4 (SEQ ID NO: 8+SEQ ID NO: 11) and cat3+GntKUR3 (SEQ ID NO: 10+SEQ ID NO: 9) primer pairs were digested with BgIII and PstI enzymes, respectively, to verify the authenticity of the gntU deletion of these strains. Expected digestion patterns were obtained with both enzymes.

The No. 2 transformant was named MDP41U2 and further used in experiments.

Removal of the Chloramphenicol Marker (CmR) from MDP41U2.

The CmR marker from strain MDP41U2 was removed by bacteriophage P1Cre recombinase using the procedure disclosed in Palmeros et al. (2000) Gene 247:255-264. Briefly, plasmid pJW168 expressing the IPTG-inducible Cre recombinase was introduced into MDP41U2 strain by electroporation, The removal of CmR marker was achieved by screening colonies for chloramphenicol sensitivity (CmS) phenotype on antibiotic-free LA plates followed by PCR verification. Finally plasmid pJW168 was eliminated from the CmS strain by growing the strains at non-permissible temperature and checking for carbenicillin-sensitivity (CarbS) of the strains. The marker-less gntU deletion strain was named MDP41UF and was used for adding the idnT deletion.

TABLE 1 Primers used for gntU Deletion Name Sequence (5′ to 3′) SEQ ID NO: GntKUF1 TTGCTGCATCAACAGTCGGAG 5 GntKUR1 GTTATTCCGGCACAAGCAGCC 6 GntKUF3 GCGCTGGCCACGGTAGTTA 7 GntKUF4 CGAAGAAGCCGCCAGGATGTTA 8 GntKUR3 CCGGCGGTATTATTTTGTTTCTGA 9 Cat3 GAAAGTTGGAACCTCTTACGTGCCG 10 Cat4 ACAACAGTACTGCGATGAGTGGCAG 11

Example 2 Inactivation of the idnT ORF Encoding a Gluconate Transporter in Pantoea citrea

Construction of the idnDT Deletion Plasmid:

Primers PcldnT-1 (SEQ ID NO: 12) and PcldnT-2 (SEQ ID NO: 13) were designed approximately 1 kb upstream and 1 kb downstream of the idnT gene (SEQ ID NO: 3). The amplification of PCR product of 1473-bp was obtained and cloned into pUniBluntV5-His-Topo (Echo Cloning System from Invitrogen). The resulting plasmid was transformed into E. coli PIR1 strain (Invitrogen, Carlsbad, Calif.) and appropriate restriction enzymes were used to check for the correct orientation of the gene.

IdnT gene was interrupted using BgII enzyme followed by insertion of a cat-loxP (1080-bp) cassette (Palmeros et al., (2000) Gene 247:255-264) resulting in plasmid pEkldnTCat2 (4813-bp). As described in example 1, the orientation of the cat-loxP cassette was checked with appropriate restriction enzymes.

The following 2 primers PcldnD5 (SEQ ID NO: 14)+PcldnD6 (SEQ ID NO: 15) were used to amplify the IdnD gene which is upstream of the idnT gene. A 1891-bp fragment encompassing the IdnD gene was amplified by PCR using known techniques and was cloned into pUniBluntV5-His-Topo (Echo Cloning Systems, Invitrogen) resulting in a plasmid called pldnD56-1.

Construction of pEkldnD56-1 and pEkldnTCat2 to Obtain the Final Plasmid pEkldnDTCat2.

pEkldnTCat2 was digested with BssH II and NsiI enzymes to remove a 1883-bp region of R6k ori and partial Km resistance gene, and the remaining 2930-bp of IdnT gene with the cat-loxP cassette was purified and ligated to IdnD gene as follows:

pldnD56-1 was digested with BssH II and NsiI enzymes and a 2576-bp fragment containing the IdnD gene with R6K ori region was purified and ligated to 2930-bp of IdnT gene with the cat-loxP cassette followed by transformation into E. coli PIR1 strain to generate the final plasmid pEkldnDT Cat2 (5506-bp) that was used for idnDT deletion. Plasmid pEkldnDT Cat2 was verified with NcoI+Bgl II enzymes.

Transformation of pEkldnDT Cat2 into P. citrea MDP41UF Strain.

The plasmid pEkldnDTCat2 was transformed into P. citrea MDP41U2 strain using the same procedure as described above in example 1 and transformants were selected on LA+Cm12. Hundreds of CmR colonies were obtained followed by picking and patching simultaneously onto LA+Cm12 and LA+Kan50 plates to screen for kanamycin-sensitive (Kan^(S)) clones. Seven (Cm^(R)Kan^(S)) colonies were checked by PCR for the deletion of the idnDT region with appropriate primers as described herein and one colony was subsequently named MDP41UDT. (FIG. 8)

Verification of the idnDT Deletion in MDP41UDT:

A set of outside primers (PcldnD7 (SEQ ID NO: 17)+PcldnT4 (SEQ ID NO: 16)) were used to verify the idnDT deletion. Another set of outside primers plus one cat gene specific primer (pcldnD7 (SEQ ID NO: 19)+Cat4 (SEQ ID NO: 11)) & (PcldnT4 (SEQ ID NO: 16)+Cat3 (SEQ ID NO: 10)) were used to verify the junctions of the deleted regions with the cat-loxP region.

Plasmid pD92-A (14.2-kb) was derived from plasmid pD92 (12.6-kb). pD92 was constructed by cloning the structural gene for the 2,5-diketo-D-gluconate reductase B from Corynebacterium sp. (Grindley et al., (1988) Appl. Environ. Microbial. 54:1770-1775) into p143 (U.S. Pat. No. 5,008,193). The structural gene for permA gene described in WO 02/12468, WO 02/12528 and WO 02/12481 was introduced into the unique NotI site of plasmid pD92. This site is located in the structural gene of the streptomycin resistant (Strep^(R)) gene and the cloning of permA caused its inactivation. In plasmid pD92-A, the expression of permA gene is driven by its native promoter. Plasmid pD92-A was then transformed into MDP41UDT strain, resulting in MDP41UDT/pD92-A.

TABLE 2 Primers used for IdnT gene deletion Name Sequence 5′ to 3′ SEQ ID NO: PcIdnT-1 TAGGTATAGCCGAAGGGATGACAC SEQ ID NO: 12 PcIdnT-2 AGAGCCTTTGCCTTTGATAACAGC SEQ ID NO: 13 PcIdnD5 CTTTGGCCGCTGAACTGACGAGAT SEQ ID NO: 14 PcIdnD6 TATAAACAGAAAAGGACAGATGAG SEQ ID NO: 15 PcIdnT-4 TACCAGCCGCATACCGATACACC SEQ ID NO: 16 PcIdnD7 CCGGTTATTCGCGTTATGTG SEQ ID NO: 17 Cat3 GAAAGTTGGAACCTCTTACGTGCCG SEQ ID NO: 10 Cat4 ACAACAGTACTGCGATGAGTGGCAG SEQ ID NO: 11

Example 3 Shake Flask Experiments

Three strains of Pantoea citrea were used in this example:

-   -   (1) Strain 139-2A/pD92 was used as a control.     -   (2) Strain MDP41/pD92A disclosed in WO 02/081440 which includes         an interruption in glkA.     -   (3) Strain MDP41UDT/pD92-A (as described above in examples 1         and 2) which includes an interruption in glkA and a deletion in         gntU and idnT.

These strains were grown in the following medium (KH₂PO₄ (12.0 g/L); K₂HPO₄ (4.0 g/L); MgSO4.7H₂O (2.0 g/L); Difco Soytone (2.0 g/L); Sodium citrate (0.1 g/L); Fructose (5.0 g/L); (NH₄)₂SO₄ (1.0 g/L); Nicotinic acid (0.02 g/L); FeCl₃.6H₂O (5 mL/L of a 0.4 g/L stock solution) and Trace salts (5 mL/L of the following solution: 0.58 g/L ZnSO₄.7H₂O, 0.34 g/L MnSO₄.H₂O, 0.48 g/L Na₂MoO₄.2H₂O) with fructose as the sole carbon source or with a mixed carbon source (fructose, gluconate and DKG) in the range from 0.1% to 1.0%. Cells were grown at a temperature between 20-37° C., (preferably below 30° C.) and at a pH 7.0 (but a pH range of 5.0 to 8.0 should also work).

Gluconate uptake biochemical assay:—Samples of the fermentation broth containing cells of the strains designated above were withdrawn from the respective shake flasks and were quenched on ice-water bath. The fermentation broth was centrifuged at refrigerating conditions and the supernatant discarded. The cell pellet was washes with 0.95% ice cold saline solution followed by two washed with gluconate uptake assay buffer (100 mM ice-cold potassium phosphate, pH 6.9 containing 10 mM fructose). Cells were suspended in the same assay buffer to an optical density of 12 at 550 nm, and then were incubated at room temperature and preferably 28° C. The gluconate uptake assay was started by mixing the cell suspension solution with C¹⁴ enriched radio-isotoped gluconate. The time course of gluconate uptake was performed using vacuum/filter based quenching using ice-cold gluconate uptake assay buffer. Gluconate uptake measurements were done by radioisotope incorporation in the cells and the data obtained plotted against time to give the gluconate uptake rate. (See FIG. 9).

The results support that deletion of gntU and idnT were effective in lowering the gluconate uptake rate. In this experiment, the rate of gluconate uptake rate for the control (139-2A/pD92) and the strain MDP41/pD92-A were similar. The gluconate uptake rate of MDP41UDT/pD92-A which includes the interruption of glkA, deletion of gntU and deletion of idnT further reduced gluconate uptake rate by 70% compared to the control.

Example 4 Fermentor Experiments with Pantoea citrea Host Cells Having a Deletion of gntU and idnT

Seed Train: A vial of culture containing the indicated strains which was stored in liquid nitrogen, was thawed in air and 0.75 mL was added to a sterile 2-L Erlenmeyer flasks containing 500 mL of seed medium. Flasks were incubated at 29° C. and 250 rpm for 12 hours. Transfer criteria is an OD₅₅₀ greater than 2.5.

Seed flask medium—A medium composition was made according to the following: KH₂PO₄ (12.0 g/L); K₂HPO₄ (4.0 g/L); MgSO4.7H₂O (2.0 g/L); Difco Soytone (2.0 g/14; Sodium citrate (0.1 g/L); Fructose (5.0 g/L); (NH₄)₂SO₄ (1.0 g/L); Nicotinic acid (0.02 g/L); FeCl₃.6H₂O (5 mL/L of a 0.4 g/L stock solution) and Trace salts (5 mL/L of the following solution: 0.58 g/L ZnSO₄.7H₂O, 0.34 g/L MnSO₄.H₂O, 0.48 g/L Na₂MoO₄.2H₂O).

The pH of the medium solution was adjusted to 7.0±0.1 unit with 20% NaOH. Tetracycline HCl was added to a final concentration of 20 mg/L (2 mL/L of a 10 g/L stock solution). The resulting medium solution was then filter sterilized with a 0.2μ filter unit. The sterile medium was added to a previously autoclaved flask.

Production Fermentor—Additions to the reactor vessel prior to sterilization included: KH₂PO₄ (3.5 g/L); MgSO₄.7H₂O (1.0 g/L); (NH₄)₂SO₄ (0.92 g/L); Mono-sodium glutamate (15.0 g/L); ZnSO₄.7H₂O (5.79 mg/L); MnSO₄.H₂O (3.44 mg/L); Na₂MoO₄.2H₂O (4.70 mg/L); FeCl₃.6H₂O (2.20 mg/L); Choline chloride (0.112 g/L) and Mazu DF-204 (0.167 g/L) an antifoaming agent.

The above constituted media was sterilized at 121° C. for 45 minutes. After tank sterilization, the following additions were made to the fermentation tank: Nicotinic acid (16.8 mg/L); Ca-pantothenate (3.36 mg/L); Glucose (25 g/L) and Fructose (25 g/L).

The final volume after sterilization and addition of post-sterilization components was 6.0 L. The so prepared tank and medium were inoculated with the full entire contents from seed flasks prepared as described to give a volume of 6.5 L.

Growth conditions were at 29° C. and pH 6.0. Agitation rate, back pressure, and air flow are adjusted as needed to keep dissolved oxygen above zero. When the sugars initially batched into the medium have been exhausted, a fed-batch process as previously described herein is employed. In this example both glucose and fructose are used as substrates in contrast to the standard fed-batch process where one substrate is employed.

As observed in Table 3 below, the yield of KLG obtained in MDP41UDT/pD92-A was greater than the yield obtained in the control MDP41/pD92-A.

TABLE 3 Fermentation Yield Performance Metrics for Yield on KLG on Sugars Average Yield Standard Maxi- Mini- Strain (g/g) Deviation mum mum Number MDP41/pD92-A 0.601 0.021 0.636 0.581 8 MDP41UDT/pD92-A 0.643 0.009 0.654 0.630 5 t-test 0.001 MDP41/pD92-A is the control strain and includes a glkA interruption. Strain MDP41UDT/pD92-A is derived from MDP41/pD92 and includes a glkA interruption, a gntU deletion and an idnT deletion.

A further unexpected result included the increased production of KLG in relation to the formation of IA. KLG was measured as described above in the general experimental methods. Typically KLG is reduced further within strains to produce idonic acid. After fermentation is complete, air can be circulated through the fermentor and a membrane bound dehydrogenase oxidizes the IA back to KLG. With the deletion of gntU and idnT, the ratio of KLG to IA is increased thus reducing the time required for the back conversion of IA to KLG (Table 4). Reference is also made to FIG. 2.

TABLE 4 Ratio of KLG to IA KLG IA STRAIN (G/L) (G/L) RATIO MDP41/PD92-A 189 75 2.52 MDP41UDT/PD92-A 214 47 4.55 

1. A method for enhancing the level of production of an ascorbic acid (ASA) intermediate comprising a) obtaining an altered bacterial host cell which is capable of producing an ASA intermediate, wherein an endogenous gluconate transporter gene has been rendered non-functional; b) culturing the altered bacterial host cell under suitable culture conditions in the presence of a carbon source to allow the production of an ASA intermediate; and c) obtaining the ASA intermediate, wherein the level of production of the ASA intermediate in the altered bacterial host cell is enhanced compared to the level of production of the ASA intermediate in a corresponding unaltered bacterial cell grown under essentially the same culture conditions.
 2. The method according to claim 1 further comprising isolating the ASA intermediate.
 3. The method according to claim 1, wherein the ASA intermediate is selected from the group consisting of gluconate, 2-keto-D-gluconate (2-KDG), 2,5-diketo-D-gluconate (2,5-DKG), 2-keto-L-gulonic acid (2-KLG), 5-keto-D-gluconate (5-KDG), and L-idonic acid (IA).
 4. The method according to claim 3, wherein the ASA intermediate is 2-KDG.
 5. The method according to claim 3, wherein the ASA intermediate is 2-KLG.
 6. The method according to claim 1, further comprising converting the ASA intermediate to erythorbic acid.
 7. The method according to claim 1, wherein the endogenous gluconate transporter gene encodes a gluconate transporter having the amino acid sequence of a) SEQ ID NO: 2, b) a sequence having at least 40% sequence identity to SEQ ID NO: 2, c) SEQ ID NO: 4 or d) a sequence having at least 80% identity to SEQ ID NO:
 4. 8. The method according to claim 1, wherein two endogenous gluconate transporter genes are inactivated, wherein a first gluconate transporter gene encodes a gluconate transporter having at least 90% sequence identity to SEQ ID NO: 2 and a second gluconate transporter gene encodes a gluconate transporter having at least 90% sequence identity to SEQ ID NO:
 4. 9. The method according to claim 1, wherein the altered bacterial host cell is a Pantoea cell or an E. coli cell.
 10. The method according to claim 9, wherein the Pantoea cell is a P. citrea cell.
 11. The altered bacterial host cell obtained according to claim
 1. 12. The method according to claim 9, wherein the endogenous gluconate transporter gene encodes a gluconate transporter having the amino acid sequence of SEQ ID NO: 2, SEQ ID NO: 4 or a sequence having at least 80% identity to either SEQ ID NO: 2 or SEQ ID NO:
 4. 13. The method according to claim 1, wherein the carbon source is selected from the group consisting of glucose, gluconate, fructose, galactose and combinations thereof.
 14. The method according to claim 1, wherein the endogenous gluconate transporter gene has been rendered non-functional by deletion of the gene.
 15. The method according to claim 1, wherein the altered bacterial host cell further comprises an inactivated endogenous glucokinase gene.
 16. A method for obtaining an improved bacterial strain for the production of an ascorbic acid (ASA) intermediate comprising a) obtaining a bacterial host cell which is capable of producing an ASA intermediate; b) inactivating at least one endogenous gluconate transporter gene, wherein the endogenous gluconate transporter gene encodes a gluconate transporter having the amino acid sequence of at least 40% sequence identity with SEQ ID NO: 2 or at least 80% sequence identity with SEQ ID NO: 4; and c) culturing the bacterial host cell under suitable culture conditions in the presence of a carbon source to allow for the production of ASA intermediates.
 17. The method according to claim 16, wherein the endogenous gluconate transporter gene has been deleted.
 18. The improved bacterial strain obtained according to the method of claim
 16. 19. The improved bacterial strain obtained according to the method of claim
 17. 20. The improved bacterial strain of claim 18, wherein said strain is a Pantoea strain.
 21. An altered Enterobacteriaceae strain comprising one or more inactivated endogenous gluconate transporter proteins, wherein the one or more inactivated endogenous gluconate transporter proteins are selected from the group consisting of the amino acid sequence of SEQ ID NO: 2, an amino acid sequence having at least 80% sequence identity with SEQ ID NO: 2, the amino acid sequence of SEQ ID NO: 4 and an amino acid sequence having at least 80% identity with SEQ ID NO:
 4. 22. The altered Enterobacteriaceae strain of claim 21, wherein the one or more inactivated endogenous gluconate transporter proteins include two inactivated gluconate transporter proteins.
 23. The altered Enterobacteriaceae strain of claim 21, wherein the one or more inactivated endogenous gluconate transporter proteins is rendered nonfunctional by deletion of the genes encoding the one or more endogenous gluconate transporter proteins.
 24. The altered Enterobacteriaceae strain of claim 21, wherein said Enterobacteriaceae strain is selected from the group of Pantoea, Gluconobacter, Erwinia, Klebsiella, and Escherichia strains.
 25. The altered Enterobacteriaceae strain of claim 24 further comprising an inactivated endogenous glucokinase gene, said inactivation preventing the expression of an active glucokinase.
 26. A recombinant Pantoea strain comprising a) a first inactivated endogenous gluconate transporter gene, wherein the first gluconate transporter gene prior to inactivation encoded a gluconate transporter having at least 90% sequence identity with SEQ ID NO: 2, b) a second inactivated endogenous gluconate transporter gene, wherein the second gluconate transporter gene prior to inactivation encoded a gluconate transporter having at least 90% sequence identity with SEQ ID NO: 4, and c) an inactivated endogenous glucokinase gene.
 27. The recombinant Pantoea strain of claim 26, wherein the first inactivated endogenous gluconate transporter gene and the second inactivated endogenous gluconate transporter gene have been deleted.
 28. A method for increasing the availability of gluconate for oxidative pathway production of an ascorbic acid (ASA) intermediate comprising a) obtaining an altered bacterial host cell, wherein an endogenous gluconate transporter gene has been rendered non-functional; b) culturing the altered bacterial host cell under suitable culture conditions in the presence of a carbon source to allow the production of an ASA intermediate; and c) obtaining the ASA intermediate, wherein the ASA intermediate is selected from the group consisting of 2-keto-D-gluconate (2-KDG), 2,5-diketo-D-gluconate (2,5-DKG), 2-keto-L-gulonic acid (2-KLG), 5-keto-D-gluconate (5-KDG) and L-idonic acid (IA) and the endogenous gluconate transporter gene encodes a gluconate transporter having the amino acid sequence of SEQ ID NO: 2, SEQ ID NO: 4 or a sequence having at least 80% identity to either SEQ ID NO: 2 or SEQ ID NO:
 4. 29. The method according to claim 28, wherein the altered bacterial host cell is a Pantoea cell or an E. coli cell.
 30. An isolated polynucleotide encoding a gluconate transporter having the amino acid sequence of a) SEQ ID NO: 2, b) a sequence having at least 40% sequence identity with SEQ ID NO: 2, c) SEQ ID NO: 4 or d) a sequence having at least 80% sequence identity with SEQ ID NO:
 4. 31. The isolated polynucleotide of claim 30 having the nucleic acid sequence of SEQ ID NO: 1, SEQ ID NO: 3 or a sequence having at least 80% identity to either SEQ ID NO: 1 or SEQ ID NO:
 3. 32. The isolated polynucleotide of claim 30 encoding the gluconate transporter having at least 95% sequence identity with the amino acid sequence set forth in SEQ ID NO: 2 or SEQ ID NO:
 4. 33. A gluconate transporter having the amino acid sequence as set forth in SEQ ID NO: 2, SEQ ID NO: 4 or an amino acid sequence having at least 80% identity to either SEQ ID NO: 2 or SEQ ID NO:
 4. 